Going to state this now so that this doesn't become some sort of flame war: I'm not trying to make this tabletop RPG more like a video game by incorporating achievements. I just like the idea of giving my players tiny little rewards for doing things that aren't usually done under normal circumstances.
Now as for my question: Can any of you think of some good achievements? I think I'd like to keep them a secret and only tell my players they've earned the achievement once they've actually earned it. And I don't really want any ideas for achievements that require bookkeeping. None of that "Get 50 kills with a plasma gun" crap. I'm just talking about impressive acts. Here's some examples.
Kill an enemy with his own weapon
Kill an enemy with your bare hands
Kill an enemy with a weapon you're not proficient with
Destroy an enemy vehicle with a hand-held weapon (not vehicle mounted)
Deal over 100 damage in a single hit (via Righteous Fury)
Deal over 500 damage in a single round (via Righteous Fury)