http://www.aperturescience.com/glados@home/ Basically, they've made a deal: buy this bundle of indie games, and if enough folks do so, they'll kick the launch date of Portal 2 up.
Now, this isn't completely altruistic of course, Valve makes Portal but also makes Steam the marketplace where you'd be buying this bundle of games. But, still, it's kind of a classy act IMO.There are plenty of ways to promote their services and their latest game, but few that would also result in this kind of good karma.
Pretty nice Valve...pretty nice.