http://gumstix.com/index.html - ~$140
Gumstix, inc. makes the world's smallest full function, open source computers. These computers and related products are marketed to companies, product designers and hobbyist in more than forty countries worldwide.
http://beagleboard.org/hardware - $149
The USB-powered Beagle Board is a low-cost, fan-less single board computer that unleashes laptop-like performance and expandability without the bulk, expense, or noise of typical desktop machines.