I'm uncertain how old this is, but it was just bought to my attention. C64s.com is a site that lets you play all those old C64 games in the browser. All you need is the Java plugin (which you most likely already have) and you are ready to rock'n'roll. The list of games is very comprehensive and screen shots are provided.

I just wasted some time on Commando, a game that had me so addicted once that it caused me to skip my homework and consequently got my C64 confiscated from me.

Now we are all grown up, enjoy without any fear.