I''m writing a 3-D game, based on the quake2 engine, and I want to know which map editor lets you use textures from individual image files, instead of those wad and pak files used in games like quake2 and half-life. Thanks in advance.

QuArK will let you use individual textures (in the .wal format if i remember correctly). Have a google for Wally if you need something for creating the textures in the correct format.