With Epic dropping it's Unreal 3 Editor on the public as a pack-in with the Gears of War for PC release Valve didn't want to be left out - so they dropped a little Matrix magic on the game development community as well in the form of their updated Source SDK.  Want to add more Portals to Portal? Done.  Want to make some mini-games featuring d0g?  Done.  Want to redo Team Fortress 2 with all the old models?....er...um..scratch that one.  But you get the idea.

Included in the Source SDK are:
• The Hammer World Editor, which provides the capability of creating custom levels for Source engine-based games
• Valve's custom Faceposer tool that allows users to produce choreographed sequences in the Source engine
• A model viewer that allows artists to preview models and their animations quickly and easily
• Source code that allows mod makers to create their own game based on Half-Life 2 or Half-Life 2eathmatch
• All of the tools required to make build custom textures and models for Source engine games

Valve also promises a future Source SDK update that will provide tools and source code for those interested in making full-fledged single-player or multiplayer mods.  Let the revolution begin!

Hop on over to Valve's developer wiki for a peek at the Source SDK Release Notes.