MAME now indexes by internal driver or system type:
: While they don't host commercial games, the MAMEDev Legal Page offers a small index of free, legally distributed ROMs for testing and personal use. index of mame roms
: Widely considered one of the safest and most comprehensive sources, the Internet Archive hosts massive "ROM sets" categorized by MAME version numbers. MAME now indexes by internal driver or system
http://example.com/roms/mame/