Useful sites
TWHL - The Whole Half Life
Is the only site you will ever need! This web site is probably the most active English site about mapping I know off. On it you will find anything you need about mapping. Examples, tutorials, fast help, technical information, and everything else! The users a very nice and will help any one who seeks help.
The 303
Is the home of an popular name in Valve mapping world. Get over there and enjoy an abundance of various indepth tutorials on texturing , modeling, mapping and more, rarely found elsewhere.
Sven COOP forums
Is a forum for the popular Sven COOP mod. On it you can find lots of useful information and catch up with the latest development of SHLT and VHLT tools.
http://www.svencoop.com/forums/
Optimization examples
This web page contains optimization info about skyboxes and skip/hint. Its for Source engine, but the difference is minimal. (GS doesn't have func_detail)
http://optimization.interlopers.net/index.php?printable
Tommy of Escondido's Mapping help
It happened to all of us. We were happily mapping and working, than all of a sudden... Something broke (hopefully not your keyboard/mouse), stuff doesn't work? Check all about errors and useful mapping informations on Tommy's web site: