Radiant
Radiant is a game level editor for Quake Engine based games originally developed by id Software and Loki Software.
GtkRadiant
An off shoot of the program Q3Radiant, GtkRadiant is able to create levels for GoldSrc engine based games (Half-Life 1, Counter-Strike, Team Fortress Classic) as well. GtkRadiant is open source, licensed under the GPL, and is available for free.
Note: GtkRadiant 1.5 has certain compatibility and stability issues when creating maps for Half-Life, for this reason 1.4 or earlier are recommended instead.