DrGargor's Skin Laboratory - The good Doctor's site has very useful tutorial for the beginning skin artist and his own skins for download.

GFE - A utility for converting *.m8 files to *.bmp format and back. It also handles many other file formats from numerous other popular games, including quake PAK files.Preferred choice of many skinners.

Wydraz's Skinning Tutorial - Very thorough skinning tutorial with step by step instructions and diagrams.

Heretic Hides - Tons of expertly done models and fun skins. Specializes in adapting Heretic II monster/NPC skins to Corvus & Kiera models.