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.