Editing

Blast's 3D gaming and editing page
Information and tutorials about how to create 3d game levels and mods specific for this game
http://blasts_pad.tripod.com/

Func_tom Half-Life Editing
With tutorials, tools, links, and forums.
http://func_tom.tripod.com/

The Graveyard
Level design tutorials, prefabs, and maps.
http://members.tripod.com/johnzombie/

Half-Life Department of Engineering
Level design and map beta testing.
http://www.game-edit.org/

Handy Vandal's Almanac
Index of design tutorials.
http://www.karljones.com/halflife/almanac.asp

69th Vlatitude
Level design tutorials and resources.
http://www.vlatitude.com/

Wavelength
Tutorials and information covering level design, coding, modeling, and sound creation.
http://planethalflife.gamespy.com/View.php?view=HostedSites.List&category_select_id=9