HL2 Deathmatch Sky List
This is a list of skies available in the Steam caches, along with pictures of each skybox. The light_environment settings listed (Ideal Sun Angle, Ideal Brightness, etc) are suggestions based on settings used in official maps; other values may also be appropriate. To test skies in singleplayer without recompiling the map, bring up the console and enter sv_skyname <sky_name>. Quicksave the game, load it again and the old skybox should be temporarily replaced with the new one. Only outdoor maps that have both skyboxes and light_environments are referenced by this list.
For a Full list of skyboxes, see this Page