Here is a collection of Links for some old classic games I still love to play...

DooM, Heretic and Hexen

Doomsday the best Doom-engine port that is around the web, for Windows, Linux and Mac OS.
Doomsday Links A nice webpage to upgrade your Doomsday to HiRes.
jDoom Resource Pack (jDRP) Make your Doomsday Doom 3d! The most popular collection of 3D Models for Doomsday.
NewDoom Good site when you look about news from the classic and modern doomworld.
Doomworld They review every week a new bounce of classic Doom addon levels as well as having good news.
Doom Wad Station Best DooM Addons page along the web.
Lord Lief's ID gameing links All patches for the old Doomgames. For case you need to patch your Doom to run Doomsday Like I had too.

Duke Nukem 3D and Shadow Warrior

3Duke32 One of the best Duke Nukem 3D Ports around.
High Resolution Pack Increase the detail of Duke3d.
JFSW Only known port for Shadow Warrior.

Quake 1/2 and Hexen2

Unoffical Quake-Engine Powerful Port to run all popular Quake and Hexen 2 Engine based games.
Vengeance R2 Best looking Quake1 port, great choose for single player gaming.
Dark Place Best online working Quake 1 port, coming with a intressting mod as well.
Knightmare's Quake2 Stuff Developer of a very nice Quake2 Port that works great online.
Quake2 Evolved Best looking Quake2 port great choose for good singleplayer experinces.

Other Classics

NewWolf Port for Wolfenstein 3d.
Birger's WinRoTT A Rise of the Tread Port.