Valve has confirmed that the Software Development Kit for its Source engine will be made available to the mod community when Half-Life 2 goes on sale, on November 16th.
Eagle-eyed observers ferreting around in the cache file of the recent Steam platform update found information relating to the tools that will be made available in the SDK; including Hammer, Half-Life Model Viewer, Face Poser, Scene Manager, content compiling tools, Documentation as well as Source code and examples. Valve later confirmed the tools will be made available - for free - to anyone who wants them when the game ships; although few people will actually have the technical know-how to put them to good use.
The news won't be of too much interest to the casual gamer, of course, but the mod community will be excited to finally try their hand at creating Source-based content; and if previous results are anything to go by we could be in for some interesting developments. Apparently, the SDK will also be updated over time via Steam automatically.