AdvMAME works fantastic for me, I've compiled it from source and that took about an hour or so to do. After that I edited the advmame.rc file to change the device_keyboard variable to raw, and installed PiKeyD. I then managed to get a button hooked up to a Pi's GPIO and set as the coin slot.
PCSX-Rearmed works really too, the audio stutters a bit, but the games run fine. Again, you need to compile that which takes a fair while, not difficult, just time consuming.
If you want something that is as simple as a single command to install, DOSBox works flawlessly right through apt-get.
And finally, if you want all of these and NES, SNES, Amiga, GameCube, GameBoy and many others, download the RetroPie script, it automatically compiles and installs all of the emulators I mentioned (except DOSBox, but that's simplistic) and also installs EmulationStation, a neat little menu front end so that you can access all of your games from all of your emulators.
For further reading, I recommend:
http://www.raspberrypi.org/phpBB3/viewt ... 78&t=29962
http://www.raspberrypi.org/phpBB3/viewt ... 00#p307100
http://elinux.org/RPi_Low-level_peripherals
https://github.com/petrockblog/RetroPie-Setup