Wikipedia has a nice long list of open source games, linking to their description, often including videos. It will keep you well busy over the free time. A personal recommendation is UfoAI (Alien Invasion), a great 3D round based first person shooter and infrastructure building combination:
It's fascinating to first build your infrastructure and later venture inside it fighting of invading aliens. Of course first you have to select and train your people, research and build of steal guns and ammo, etc. But be careful, the game can be quite addictive.
If you liked this post, you might want to link to it and subscribe to the RSS feed so you won't miss the next one. In any case, check the related posts below. Maybe I'm just having a really bad day and normally write much more interesting posts about theses subjects. Or maybe you'll only understand what I meant here once you've read all my other posts on the topic. ;-)