I found some browser-based game emulators worth noting. I haven’t tried them all out yet.
DOS games: http://dosdose.com
8-bit Nintendo games: http://nintendo8.com/
Commodore 64 games: http://c64s.com/
4-bit Gameboy games: http://gbemul.com/
Sega Master System games: http://mastersystem8.com/
Apple II games: http://www.virtualapple.org/