Serious Sam: Double D
It's only a 2.5D side-scrolling platform shooter, but somehow Double D managed to capture the intensity and wackiness of the Serious Sam FPSs. Hordes of well-known monsters from the 3D games and new baddies of all sizes, power-ups, secrets, it has it all. And lo and behold, the Gun Stacker system lets you combine and use multiple weapons as one massively insane superkillertool.
A demo is available on Steam.
