Source: HalfLife2.net

HalfLife2.net reports that with the update of Steam last night, also came a file containing information on the contents of the soon to be released SDK for Half-Life 2:

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.

Included:

-Hammer
-Half-Life Model Viewer
-Face Poser
-Scene Manager
-Various content compiling tools
-Documentation
-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.

For more check out HalfLife2.net.