When I do play emulators, I use these:

FCEUltra for NES
ZSNES for NES
VBA for GB and GBA
And one called Stella for real old-school Atari 2600. Yes, I still play those on the odd occassion.

Are there any good PS2 emulators out there? My PS2 died a few weeks ago and I'd rather not have to spend the money to replace it.