Griffin PowerMate: http://www.thinkgeek.com/computing/accessories/5ca2/
Note that I have no clue if this is any good for games at all.
It just looks interesting.
I don't know if the software is up to the job, it says it can be used to send key commands, which may or may not work.
MAME is mentioned in this review for example and it sounds good: http://gear.ign.com/articles/376/376480p1.html
And some criticism here: http://www.gameex.info/forums/index....26&#entry16226
Just google for "Griffin PowerMate" and MAME or any of the typical candidate games (Arkanoid, Tempest, etc.). Plenty of discussion on the subject apparently.