Get it while it's hot from: Source Updates.
I'll also mention there are a couple of new interesting wiki articles: one from R. Belmont on how tilemaps work, and one from Mr. Do! on how to write .lay files for the artwork system. If anyone would like wiki access to write an article, use the contact form to send a proposal and request to the team. If you haven't checked out the wiki in a while, you may find there's a lot more content there than there used to be!