I'm not a grammar expert by any means, but there are a few misuses that leave me feeling a little perturbed. I left out some of the most common offenses, such as your/you're, there/their/they're, to/two/too and it's/its because they should be implied.
1. Ironic: characterized by often poignant difference or incongruity between what is expected and what actually is (not to be used interchangeably with coincidental)
2. Toward: no "s" on the end
3. "I could care less" this really makes no sense; it should be the negative "I couldn't care less"
4. Nonplussed: to render utterly perplexed; puzzle completely (often misused to mean just the opposite, e.g. unimpressed)
5. i.e versus e.g.: i.e. from the Latin, meaning "that is"; e.g. also Latin, meaning "for example"
6. Affect: to act on; produce an effect or change in; to influence (whereas an effect is produced by an agency or cause - a result)
7. Capital: is the city, but Capitol is the building
8. Complement: either of two parts or things needed to complete the whole; counterpart; red and green; mustard and ketchup (a compliment is an expression of praise, commendation, or admiration; mints or chocolate)
9. Penultimate: next to the last (often misused to mean the last)
10. Dawn: the first appearance of daylight in the morning; antonym: twilight (often misused to be interchangeable with sunrise)
