FreedroidRPG ( www.freedroid.org )

FreeDroidRPG is literally my favorite open source game of all time.
It's an action RPG, it's VERY Diablo esque, but in a futuristic setting and with guns.
You play as a tux(Because why not xD) which is supposedly a race that is natural with computers(or something like that) and thus are really good at hacking and programming, and instead of spells you have programs that you use on the enemy(robots)
Really cool game, only downside is really slow development. It is being developed for more than 10 years already and there's only one act, and if you look at videos from like 2011 the game still looks and plays basically the same. So it's a bit too short, but it's still fun.

[Repost from dRCS#Showcase]