So I've been playing some old school Doom and Doom2 after reading that cool book about the John's (Carmack and Romero, not Flansburgh and Linnell). Anyway I was using this prboom engine which was quite decent. It's main feature was how very true it was to the original engine, all the way down to the 8 bit sprite limitations.
Today I happened across this killer listing of old games, and how you can still play them. In that list was Doomsday which is the coolest engine for: Doom: Doom ][, Hexen, and Heretic! Hells ya! It's got a million cool options and it looks gorgeous. Check it out!