From Valve Developer Community
Welcome to the Valve Developer Community. Whether you're building a "Triple-A" commercial project or a mod for Half-Life® 2, you'll want to sign up as a Valve developer today. It's free to join, and open to anyone working with the Source Engine and the Source SDK.
Source and the Source SDK
Our knowledgebase of technical information about using the engine and all of the related tools is posted on a community-driven web site called a wiki. Sign up as a developer to contribute.
For Mod Teams