The Aperture Science website has had a mysterious countdown on it recently -- it's just been revealed what it means.
All you need to do is start playing any of the indie games from the Potato Sack Bundle -- and encourage your friends to do the same. The more people playing the games on the list, the quicker Portal 2 comes out. You don't have to purchase the games again if you've already got them -- just play them. Don't forget the Potato Sack Bundle, an awesome deal in itself, will no longer be available once Portal 2 launches.
Can't talk now, games to play!