[mtbopfuyn]

Incorrect usage of there/their/they're and your/you're is one of my pet peeves.

then/than

[-YYZ-]

Don't forget affect/effect.

Also, a little pet peeve of mine, what's with "orientated"? The dictionary says it's valid, but to me it seems that "oriented" means the same thing and has the advantage of having one fewer syllables. Near as I can tell, through sloppy usage the noun "orientation" has resulted in the bastardized verbs "orientate", probably as a result of management types trying to sound more intelligent by adding syllables to words.