I'm a past Member of the greatest of all Level Editing Teams (IMO)....Team TNT.
Our First project was Evilution,
which is half of ID Software's Final Doom.
Our Second project was Icarus: Alien Vanguard, a single player, full 32 Level re-write.
Our Third project, Bloodlands, is a 32 Level Deathmatch Fragfest.
Both of these and more are available for free from: ftp://ftp.cdrom.com/pub/doom2/themes/TeamTNT
They require the full Version of Doom ][.
More interesting Doom & Quake related sites: