Battle for Wesnoth is a pretty cool game RPG style where you’ll build up and lead an army to conquer the entire map / world. The game has over 200 units in 16 different races so there you’ll not get bored easily.
Being open source any one has the freedom to build other maps and scenarios. You can also do the social gaming thing through its multi-player interaction. I actually enjoyed this while in college, a nice way to kill time and while interacting with other people across the globe.
One of the most active opensource games, it is available in more then 30 different languages and will run on Windows, Max OSX and several Linux distros