And to clarify--it's free as in "You download it now, you get to have it free forever" not as in, "It's free until May 24th, at which point you have to start paying for it." A good catch on Whil's part.
Please note that you don't get access to Source SDK (and by extension the level editor and various modded levels) unless you purchase some other Source game (as in pay money for it).
If I had a signature, this is where it would be...