Of the things mentioned above, I have tried:
# Knee-Deep in ZDoom (it WILL make you rage. The last level is cool while the other levels are oversized and the keyhunt is extended to 6 keys. The new critters are cheesy as fuck, one of them will insta-kill you in one hit, and not even IDDQD can save you from that.
# Scythe 2. Or maybe it was Scythe 1, don't recall. Interesting set of maps, but way too complicated levels at times, I noclipped through a lot of doors towards the end.
# The Sky May Be. You HAVE to try this, it's made by the guy who wrote "Ultima 9: Ocean travel without a boat", "A Fistful of Ultima 7" and "System Shock: A Complete Mental Breakdown". This WAD is so fucked up it's awesome.
And of course, DoomRL.
I also recommend giving the Aeons of Death mod a try: It's standard Doom with virtually every enemy and every gun from every FPS ever made.