Every year, amazing new wads are released. New ports pop up and old ports get improved. Doom is still very much alive.
Right now I'm playing through Valiant (RC).
Where?
http://zandronum.com/ comes with Doomseeker, which will find multiplayer games. I don't play it that often, but every time I've been on there's always been at least one deathmatch with 10+ players on it.