The tower defense FPS Sanctum is available to play this weekend free of charge on Steam. This free period only lasts until the end of the weekend but if you do happen to develop a taste for the unique blend of gameplay on offer you can get hold of it until the 7th of May with a 75% discount, not bad at all.
The game boasts some pretty nice features including 4-player co-op, 4 different game modes and 87 Achievements (if you’re into that kind of thing).
I gave it a whirl earlier today and was pleasantly surprised, especially considering the current price. Unfortunately it doesn’t have the largest online community but if you’re in the market for a co-op game to play with some friends it definitely deserves a chance.