MAME

Multiple Arcade Machine Emulator - a very popular (and eminently portable) arcade emulator that is the major driver for ROM swapping (around 90% of the ROMs required are either still under copyright restrictions or illegal to own unless you also own and/or maintain the original hardware).

• MAME OS X - recent Intel/Universal port
• KnoppixMAME - an all-in-one bootable CD/DVD
• MAME on P800
• AdvanceMAME - a special version targeted at hobbyists that want to run MAME on arcade monitors or TVs (some people actually rebuild original cabinets around cut-down PCs running this). Includes a graphic menu and a mini Linux distro that fits on a 185MB mini-CD.

Much to my surprise, it was even ported to the SonyEricsson P800: