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. Once registered, you’ll have unlimited access to the WIKI and discussion areas of this site, inclusion in the developer mailing list, online file hosting space for your project, and more.
Looks like it’s going to be a valuable site, for sure. It’s all based on Wiki, which is also cool. If you want to be a Source related developer, definitely check out the site here.