OK, I know this has nothing to do with roads, but besides being a road geek, I'm also quite a grammar nerd, something my wife, thankfully, appreciates about me.  As such, bad grammar bugs me, sometimes to the point of making me cringe.  With the advent of the Internet (thanks Al Gore!), we now get to see on a regular basis just how horrible many people's writing skills and associated grammar and usage really are.  Even with spell-checkers and grammar-checkers (which are not always correct and should not be relied upon), the number of grammar, spelling, and usage errors that I see on a daily basis is atrocious.  To maybe help stem that tide, as well as to make myself feel better for trying to do something about it, here is my list of the most egregious and annoying errors I see.  I know this can be a touchy topic for some people, and I don't mean to offend anyone or sound holier-than-thou, so please don't take anything below that way.  I know I'm not perfect either.  All I'm really trying to do is impress all my former English teachers.  :-)

Me and I
As a child, many of us were scolded by English teachers for saying things like Me and Johnny went to school.  It should, of course, be Johnny and I went to school.  As a result, people often use "I" in every complex subject or object.  However, there are times when using "I" is wrong and using "me" is correct.  Without getting into the technical details, the easiest way to know which to use is to drop the other person from the sentence and see how it sounds.  For instance, if you want to know whether to use "me" or "I" in the sentence Bob came with Suzy and I to the movie, drop "Suzy" and say what you're left with: Bob came with I to the movie.  That obviously sounds wrong, so try it with "me" instead: Bob came with me to the movie.  That sounds better and is indeed correct-- the rules don't change just because there's another person.  So the sentence should be Bob came with Suzy and me to the movie.  In either case, it's generally considered proper to put the other person before "me" or "I".

Us and We
In this case, I'm talking about sentences like Us men are going to the game or That is up to we employees to decide.  Are those correct?  Just like with the me/I situation above, drop the extra word after we or us and see if it works: Us are going to the game and That is up to we to decide.  Both obviously are wrong.  The correct shortened sentences are We are going to the game and That is up to us to decide, so the corrected original sentences would be We men are going to the game and That is up to us employees to decide.  Again, all you have to do is simplify the sentence and see which pronoun "sounds right".

Your and You're
"Your" is the possessive of you (What's your name?); "you're" is the contraction of "you are" (You're going to freak out.)  Here's a sentence that uses both: You're going to change your mind.  If you're not sure whether to use "your" or "you're", ask yourself if you can substitute "you are" in the sentence; if you can, then use "you're"; if you can't, then use "your".  (Here's a little reminder from Ross on Friends: http://www.youtube.com/watch?v=STYDAb_iCjg.)

There/Their/They're
"There" is the location (Go over there!); "their" is the possessive of they (What's their problem?); and "they're" is the contraction of "they are" (They're going to freak out.)  Here's a sentence that uses all three: They're going to drive their car there.

Its vs. It's
"Its" is the possessive form of it (What is its problem?); "it's" is the contraction of "it is" (It's going to rain.)  Note that "its" is the only possessive that does not use an apostrophe (see next topic.)  Here's a sentence that uses both: It's taking its sweet time.  If you're not sure whether to use "its" or "it's", ask yourself if you can substitute "it is"; if you can, then use "it's", if you can't, then use "its".

Apostrophes vis-à-vis plurals and possessives
Apostrophe misuse with plurals and possessives is rampant.  People either use too many apostrophes or none at all.  But it's not that hard to know when to use an apostrophe and when not to.  Possessives always have an apostrophe (except for its, as discussed above); plurals don't.  It's that easy.  Here's an example that uses both: What color are Bob's eyes?  There are two eyes (plural) that belong to Bob (possessive).  For singular nouns that end in an "s", you still add an apostrophe and another "s" at the end for possessives: The boss's office.  But to make a plural possessive, you generally just add an apostrophe; the other "s" is assumed: The parking lot has space for 20 customers' vehicles.  Note, however, that adding another "s" after the apostrophe in such a situation is considered acceptable, although not standard.

Decades (e.g. the 90's)
When referring to decades using numbers, the 90's is acceptable, but the '90s (the apostrophe implies that "19" is missing), or even just the 90s are preferable. 

Mute point
I guess people think the term is mute point because the point that is being made should be muted (silenced) since it's irrelevant, but the correct term is actually moot point.  "Moot" means something that is open for debate.  So when you say something is a "moot point", what you're saying is that its only relevance now is that you could debate about it.  Another way of saying it is that it's "academic" now.

Could care less
You could care less about grammar?  Really?  Then that means that you really do care about it!  After all, if you could care less about something, then you must currently care some amount about it.  What people are trying to say when they (mis)use that expression is that they don't care at all about something.  Therefore, the correct expression is could not care less, or couldn't care less.  When you say that, you're saying that you already care the least amount possible and therefore you can not care any less about it.  I think people have just gotten lazy in their speech patterns and couldn't care less has become could care less.  But it's still wrong, even if you couldn't care less.

Hung vs. hanged
When pictures are put on the wall, they are hung.  When a person is executed by dangling them from a rope, they are hanged. 

Faze vs. phase
I would like to say that people who get this wrong don't faze me, but I'd be lying!  Phase is a noun meaning "stage"; faze is a verb meaning "to bother".  Phase can also be used as a verb in the terms phase out or phase in.  Here's a sentence that uses both: I was not fazed when my job was phased out.  Don't worry if you didn't know this one, though-- my wife is a logophile and she didn't know this one until I told her!  Even the media sometime gets it wrong.

Daylight Savings Time
The correct term is Daylight Saving Time (no "s" on "Saving").  More appropriately, it should be daylight-saving time.  Why?  Because it's a time when we're saving daylight, a "daylight-saving time".  Daylight Saving Time (capitalized) is also a proper noun and can be written thusly.

Misuse of "Standard Time"
Speaking of Daylight Saving Time, this one is more obscure and not really a grammatical error, but annoying to me just the same.  Many people think that the term Standard Time means the time that is the "normal" or "standard" time in the place where they live.  So to sound official and/or "proper" when scheduling appointments or meetings, especially with people in other time zones, they'll say something like The conference call starts at 9:00 am Central Standard Time.  What they don't realize, however, is that to someone who understands these things and is pedantic, they just sound confused most of the year.  Why?  Because the term Standard Time refers to the time in a place when it's not Daylight Saving Time, in other words, the "unmodified" time.  For instance, 9:00 am Standard Time is 10:00 am Daylight Saving Time (DST).  So during DST, whenever someone tells me to call them at "9:00 am Central Standard Time", I sometimes find my self asking if they really meant 9:00 am Central Daylight Time (CDT) just to make the point.  If you want to avoid all of that, just say The conference call starts at 9:00 am Central Time.  Ninety-nine percent of the time (no pun intended), that's sufficient.  The only exception to that would be if you're in one of those weird (or perhaps enlightened) places that doesn't observe DST; then you need to be explicit or do the conversion for the person on the other end since they probably won't know how to figure it out anyway.

Who vs. Whom
Many people use whom to sound proper or even snooty, but there are only certain cases when you should use whom, and you should generally always use whom in those cases, so it's not always proper and it's never snooty.  Whom should be used when it's the object of the sentence, but that's the technical explanation.  The easiest way to know when to use whom is to ask yourself if the question you are asking (or the question form of the statement you are making) could be answered by saying "him" or "he" (I'm using the masculine form for simplicity; feel free to substitute "her" or "she" respectively.)  If the answer is "him", use whom.  If it's "he", use who.  For instance, (Who/Whom) is going to the store?  Answer: He is.  Therefore, the correct question is Who is going to the store?  Another example: (Who/whom) do I love?  Answer: I love him.  Therefore, the correct question is Whom do I love?  One last example that's a little more complicated: Bob, (who/whom) you probably remember, is back.  Change it to a question: (Who/whom) do you probably remember?  Answer: You remember him.  Therefore, the correct sentence is Bob, whom you probably remember, is back.

Irony vs. Coincidence
Ironically, the term ironic is often misused.  Most of the time when people say something is ironic, what they really mean is it was coincidental.  Irony is when something happens that would not be expected.  For instance, a fire station catching on fire and burning down is ironic.  However, someone being convicted of murder 10 years to the day after the murder happened is just plain coincidence.  There's nothing special about that day that would make you think they would or would not be convicted, so it's just a coincidence that it happened on that day.  Only use ironic when it's something that on its face would be unexpected; otherwise, use coincidental.  By the way, most of the things in Alannis Morisette's song "Ironic" are coincidental, not ironic.

Literally
Lots of people say things like I literally died when he told me that.  Really?  You literally died?  If so, then you would be dead.  When you stick the word literally into a sentence, it's supposed to mean that whatever you're saying is exactly what happened.  But the word has been literally misused by literally so many people for literally so long now that some dictionaries now literally include a definition equating the word to the real word that people mean to use: virtually.

.75¢
I always love seeing signs in restaurants or stores advertising something for ".75¢" (or ".25¢", ".50¢", etc.)  I really want to get the item in question and give the cashier a penny and tell them to keep the change.  What lots of folks forget is that .75¢ is 3/4th of one cent, so one penny would pay for it with 1/4th of a cent left over.  Of course, what they really mean is the item is 75 cents, which is correctly written either "75¢" (no decimal) or "$0.75" (3/4th of a dollar.)

Improper use of quotes for emphasis
I hate seeing signs like "Do not" park here!  The quotes around Do not are intended to emphasize those words, but that's an improper use.  Quotes are properly used to indicate spoken dialog, a direct quotation from someone, a title, an undefined term, a term used outside its normal usage, a nickname, or irony or sarcasm.  So a sign reading "Do not" park here! could be interpreted as the author either saying "do not" in a sarcastic manner, as in "do not (ha ha) park here", or that it means something not consistent with its normal meaning, for which I can't even think of an example.  Instead, the correct way to emphasize something in print is to use capital letters, bold type, italics, and/or underlining, e.g. DO NOT park here!

Improper use of all caps
Some people I know insist on writing something like My MAC gets a lot of SPAM when I access the WEB.  Neither MAC nor SPAM nor WEB are an acronym for anything, so they should not be written in capital letters unless it's for emphasis (as noted in the topic above.)  Barring that, the sentence should be My Mac gets a lot of spam when I access the web.  Mac has a capital M because it's a proper noun; the other terms should all be lower-case although Web with a capital W would also be acceptable since it's a shortened version of the proper name World Wide Web.