Marvell Semiconductors is releasing a nice little device - the SheevaPlug. Its supposed to retail around $100 and from what I can tell, it packs a serious punch.
It has 512MB of memory and 512MB of NAND flash, so it could run Debian right off the bat, and would also have pretty good performance, as it has a 1GigE network interface, and a 1.2 Ghz ARM9-like CPU core.
The best part: it only uses 5 watts of power!
I hope that devices like these continue to decrease in price as much as they have in the past few years. They could really be useful!
The device can be ordered here.
UPDATE 2: Martin shares some power ratings for the sheeva plug - they are terrific - all under 10 watts!