Also available in a 4-pack for $15.29.
Dungeon Defenders is a tower defense type game: you and up to 3 others use a combination of barriers, active defenses, and your own attacks to defend a crystal. There are 4 available classes, each with their own abilities and defenses, all combined with RPG elements like equipment and leveling up.
81/100 on Metacritic. This one is quite enjoyable to play with friends.