N64: I use Mupen64Plus. There's also Project64 out there. They're not perfect but their damn close.
PSX: There's ePSXe which has a lot of fancy features but is pretty worthless unless you've got a fancy gaming computer. pSX is basic but it emulates great and doesn't require any special plug-ins. There's also PCSX.
PS2: You're only real option is PCSX2. You'll need a good computer to run games good, though. If you have a good gaming computer, a lot of games can run at full speed.
Xbox: I'm not quite sure there's anything worthwhile. According to Wikipedia, there's cxbx, but there's only 3 games that can be played (and none of them are really worthwhile, I think). Well, if you have a Xbox or a 360, Xbox games are dirt cheap nowadays, and there's a lot of good ones out there.