Yes there are! Currently I found a Mame, Gameboy, N64, Snes, ScummVM (emulates classic adventure titles like Secret of Monkey Island, THE DIG, Day of the Tentacle, Grim Fandango and Broken Sword) and a Genesis emulator.
All of them use the PSX buttons but you need to configure all of them in the internal emulator configuration.
I have them all installed but I only tested the MAME, Sness and ScummVM emulator. I also have been told that some of these were removed from the market since they were paid apps that were copied from open source emulators. I have no idea about the legality of the use of emulators.
Since some of the APKs are not available via Market you'll need to install a file manager and allow the android system to install apps that are not from the market.
I really can't find the direct links to the emulators, but here it is their websites:
Btw: The Tiger Emulator page says that it also emulates other platforms, I've only tried Marvel vs Capcom and Metal Slug.
Oh yeah, they also run great with zero lag but they'll usually suck your battery dry.
Edit: I tested all of them and everyone works like a charm, with the exception of the N64 emulator. Some games like Mario Kart 64 run OK and other like Pillotwings64 are unplayable.
Double Edit: I've been doing some research about PSX emulators and some close friends have recommended me FPse (Free Playstation Emulator) - it's supposed to be free but someone is selling it on the Droid Market.
I haven't got the chance to test it and I'll only get my PSX games after the weekend I believe that if someone does want to emulate PSX, it seems like a good way to start.