My Open Source Game of the Year...

... is most probably OpenTTD. It's the same old Transport Tycoon kind of game - you think! Until you realize that the users have created tons of beautiful content including fully open source Grafics, Musics and Sound effects, but also new vehicles like the transrapid and tons of maps from all over the world! :)

This is one of these old style games created with lots of love and care, but
WARNING: it's highly addictive!

If you're under linux, you need to download the graphics package and unpack it into ~/.openttd. I wish the software would do that for you as well in the same way it downloads sounds, maps, extensions and everything else completely automatically!