Forbes has a list of the world's Cheap PCs, starting at $85, like the MicroClient JrSX, and the Zonbu we wrote about lovingly the other day. There's also a few paragraphs dedicated to the OLPC, which as you know ain't for public consumption right now, and an AMD powered decTOP.
The Zonbu is the most powerful on the list with its 512MB of RAM and 1.2GHz intel-compatible VIA chip, but its $99 price tag is negated by its $13 Amazon S3 subscription commitment for 2 years. Consider that a pittance for the 25 GB of online storage the Zonbu syncs your data to.
And never forget that a lot of used PCs can be had for far less. Find em at Craigslist and at the flea markets.