Stack Gun Heroes is a game about improvising ingenious guns in the middle of combat.  Check out the Steam page for more info.

You use the Stack Gun to defend a portal against hordes of infected robots trying to get off world. You'll design guns, slide up walls, collect superpowers, and earn the love of countless adoring fans in a universe teeming with robotic life.

Stack Gun Heroes is best played with 1-4 people in either solo play or co-op.

The game has been in development full time by Unstackd since the summer of 2012 so it has a lot of content.  You can mess around with the novelty of stacking or go in deep and explore everything the game has to offer.  There is plenty to do.  Many volunteer alpha testers have hundreds- or even thousands- of hours logged.