Crackdown (360)

Gosh, no, I didn't stop playing just because I'd finished killing all the gang leaders. Not even close.

I spent a few hours searching the map for agility orbs - I've still got fifty or so to find, no idea where they could be - and then turned gangs back on so I could get the Untouchable Agent achievement, which involves killing 200 enemies without dying. It was pretty easy, but great fun.