The misuse of ironic/irony always gets me. I blame Alanis Morrisette.
Another one is the word "rim" used when someone is talking about the wheels on a car. I have no idea how that started, but in no way is a car's wheel the rim of anything. It has a rim, but that's only a small portion of the wheel, and is essentially never what the person is talking about. Drives me crazy.
Then there's just the common misuse of phrases that come about because no one bothers to actually listen to themselves and think about the meaning of the words they're saying, i.e. the eggcorns - "for all intensive purposes" (intents and purposes) "I could care less" (so you do care some, then?) "supposably" (instead of supposedly) or probably the most common, the use of "of" instead of "have" or the "'ve" contraction- he should of done it vs he should've done it, etc.