The Source SDK is available free of charge via Steam with purchase of a Source-based game, such as Half-Life 2 or Team Fortress 2. Learn more about building Source-powered mods at http://developer.valvesoftware.com/wiki/category:modding.
Do you teach game design?
SourceU, a new educational licensing program, bundles Valve's game development technologies and other resources. Click here to learn more.