I saw it a few hours ago, downloaded it straight away! A word of warning though, the free-ness will only last for a few more hours and the servers are exploding! (it took me almost an hour to even get to the download button because the servers kept crashing) but once the download starts you're in.
I made an account and installed steam just for this. I'm downloading L4D2 right now. I have it on the 360, but have always wanted all the custom type of stuff and all that. Thanks OP for the heads up, cause I would probably have never known about this.
I'm so glad I didn't miss it. Got it right away and played today with my friend already :d T'was fun, but we both ragequitted, because we couldn't get to the chopper in The Parish :U We were so damn close....