This site lets you play old Commodore 64 games in your browser, without downloading any software. (Assuming your browser has Java and is on a reasonably fast machine, of course.) The experience includes everything, from SID chip sound to cracker-group intro screens, though your frame rate may vary (it feels roughly like C64 emulation on a 486-class machine in the mid-90s). A new game is added every day; today's addition is Giana Sisters, a Mario Brothers knockoff with added 1980s hairspray.
http://worldofspectrum.org/ has been doing this for ages with their entire game archive. Still, it's good to be able to do the same for C64 games...