Stops copies me!
Watch the first video in the link until ~45sec mark - that should give you the answer.That was unexpected.
I can't tell wether it will be free or not, different sources say different thing.(from the official site: AS:I is built upon the Source Engine from Valve and will be available as a competitively priced commercial game!)
Sadly won't be for mac
Can't wait to see this released though now, never even knew it was being developed as a standalone game - the UT2004 variation of Alien Swarm was awesome