I've recently installed the Doomsday Engine (http://dengine.net/) on my Mac and have been replaying through Doom and Doom 2. I also invited a friend over so we can play some COOP and deathmatch games.
Hear, hear! I replayed the old Dooms (< 3) a few years ago using Doomsday and was surprised at how fun they still were. I was afraid they'd be outdated and boring, but they really seem to tick off all the right boxes in terms of fun gameplay.
Still one of the most fun games ever.