Valve is giving us a chance to launch Portal 2 early!
http://www.aperturescience.com/glados@home/

Remember the Potato Sack? GlaDOS is apparently drawing "power" from computers playing games in the potato sack. For every collective 48 hours for one particular game, that game's "power bar" goes up by 1%. And Steam needs to record you actually playing the game - just leaving something idling in the background does not contribute.

The potatoes we all frantically collected can still be collected - in fact, you are a major contributor if you even collect one potato in one game. Potatoes multiply that game's particular power, so if, say, you gathered 5 potatoes in Audiosurf and one in Bit.TRIP BEAT, you'd be  multiplying EVERYONE's efforts in Audiosurf by 6x and BIT.TRIP BEAT by 2x.

Some people seem to think that launching multiple games is a good idea. This is in fact, not helping at all, because Steam only records keyboard activity in games launched through Steam, not through external launching.