Steam is currently running a holiday sale from today until January 2nd, 2009, with the offer extended to their collection of indie games as well. Here are some indie picks from their stable of games:

Audiosurf $4.99 (from $9.99, 50% off)
World of Goo $14.99 (from $19.99, 25% off)
Trials 2 Second Edition $2.49 (from $9.99, 75% off)
Defense Grid $14.99 (from $19.99, 25% off)
Portal $4.99 (from $19.99, 75% off)
Multiwinia+Darwinia $7.49 (from $14.99, 50% off)

Indie Games on Steam (source: Retro Remakes)