I spent most of last night playing this: http://icculus.org/neverball/
It's a pair of related open source games. Take a look at the screenshots. One is deceptively simple. You tilt a table to move a ball and knock into enough coins to escape the level. The other is a crazy golf game. Seems simple enough, doesn't it, dull even, but I bet you can't put it down.
Here's another, a Godsim this time, again 100% free to download and play: http://www.glest.org/
In Glest, you control a band of warrior magi. I'm just at the point where I can fend off an attack by having a Summoner create enough Daemons to guard my encampment, with the help of a few fire bolts. Again, utterly addictive.
There's a long list of really good games out there being written by teams of people just for the love of it. So enjoy, but don't blame me if your life falls apart, you neglect your practice and end up a penniless addict like me!

EDIT: They're for Windows, by the way...