Daredevil on CoD4 - Kill someone when you've been flashbang'd
Tactician, Mass Effect - Complete the majority of the game with more damage on shields than health - I thought it'd be really hard, but I got it on first playthrough :S
High Flyer/Base Jumper, Crackdown - Get to the top of the tower, then jump into the water below. Great fun, and awesome view from the top....I might go do it again now...
Most of the time I only get about half the achievements, because I'm neither good enough nor can I be bothered to play online, with the exception of Oblivion, as it has no online mode, and the achievements are *not* hard...They're not very imaginative, but there is a good sense of achievement when you can say that you've completed most of the main quest lines.