http://developer.valvesoftware.com

If you've ever wanted to make a mod for games using the Source Engine, the Valve Dev site is the place to go. It's full of articles contributed by everyday programmers ranging from basic programming to tutorials on Source development. The FAQ's and tutorials have nearly all the information needed to start developing your own plugins. Just thought i'd share this, most people probably have stumbled across it already but it never hurts to make sure anyone interested has access to it.