Source Software Developer's Kit
The Source SDK is a set of tools and resources used to create custom content, modifications to existing games, or even completely new games using Source engine technology. Anyone can download and try the SDK, but please note that it is designed for advanced users.
Half-Life Model Viewer
Various content compiling tools
Source code and examples
For community support and an online knowledgebase, please visit the Valve Editing Resource Center (VERC).
The SDK will be updated over time with new resources, source code, or examples. As with Steam games, these updates will be delivered by Steam automatically.
So expect this via steam and sites like Wavelength and Valve Editing Resource Center (VERC).