Are you jonesin' for some old-school 8-bit Nintendo action? Look no further than vNES. You can play 250 Nintendo games online within your browser. I could only get the controls to work for one player, but this online emulator is pretty cool. I loaded up Rygar just so I could listen to the sweet music. (Source: vNES)
UPDATE (6/8/07): The vNES site can be accessed at www.virtualnes.com. Due to some legal concerns, the games are no longer hosted directly at this site, but the author is offering vNES as a download that can be installed on any web site. A link is provided to access vNES on another "mirror" site.