There are a few good flash games out there, even if I spent quite a few years-saving on my PC to be able to play any game from 2000 on without any lag, but there are a few good gems out there.
1066 is great from my point of view, although it wears off fast after finishing their "campaign" and playing a few custom battles (it protected me of extreme boredom when I was sick before getting my PC). There's also
Reimagine, although I think someone else posted it here before. Oh yeah, and
BeGone, a superb browser FPS, which I always log on when I have to relieve stress quickly and without loading screens