Virtual Apple ][ is a site that features over 1,300 games from the Apple II and Apple IIgs days, including The Oregon Trail (pictured below), Tetris, Kings Quest, the Ultima series, Lode Runner, and many more. Unfortunately, the IIgs games require Windows and Active X, the Apple II games run in any browser through a Java-based emulator.
Hope you don't die of dysentery
Intel-based Mac owners, however, can play the IIgs games by downloading a standalone emulator linked to from the Virtual Apple ][ site. A standalone Apple II emulator is also available. The site owners said they "will soon be offering a complete download package of all disks. We also are expecting a Mac OS X compatible version of the emulator sometime soon."
Since the games run in a browser, players simply click "Switch disk" to move between virtual floppies, with buttons available to reset the game or restart the virtual computer. Unfortunately, some games work better than others, so your mileage may vary.
Thanks to Macworld for the heads-up.