Valve Software has announced they'll be making the Source Engine's Software Development Kit (SDK) available free of charge to the community and their gaming customers.
Details can be found on the Valve Software web-site
. They also have information on their Wiki
as making free access to the Source SDK should allow their "modding" community to further flourish.
This announcement comes just a few days after Valve announced they would be making Team Fortress 2 available free of charge.
Those interested in checking out the Source SDK can obtain it from the Steam client, per long as they have purchased a Source Engine game from the multi-platform client like Counter-Strike: Source or Half-Life 2.
I'm afraid I don't have time to comment further or analyze the SDK at this time due to a Phoronix trip to Washington