Just put an hour in to Paranautical Activity

EDIT: I would like to note that I don't have any affiliation with the dev's.

After playing about an hour of Paranautical Activity, I would like to sincerely recommend it to any twitch fps fan.  It's a challenging voxel styled Quake, The Binding of Isaac, and Doom inspired first person roguelike-like that's currently in beta.  It's available on desura as well as through BMTMicro on the dev's website.

Current patch's trailer.

Looks epic