This'll be a long list..
NES
N64
Gameboy Colour
PS1 (for about 2 weeks, decided it was crap and went back to the N64)
PS2
Xbox (happens to be able to emulate all the above systems except the PS2, makes a damn good media center as well once you fit a big HD)
Really wish I still had my old NES, parted with it and loads of games, lightguns, cheat thing etc for about a tenner a few years back. Now I see one with 2 games and 2 controllers for ~£40 in Gamestation last week.