The Raspberry Pi Model B is one of the most appealing mini-PC boards on sale right now. For only $35, you get an ARM-compatible Broadcom SoC, 512MB of RAM, dual USB ports, an SD slot, HDMI output, and Ethernet support all on a tiny circuit board that measures about 3.4" x 2.1". The Model B has been shipping for a while, and it's finally being joined by the Model A. This newer version has started rolling off the production line and should be available for purchase early next year.
While the Model B came first, the Model A is the one that delivers on the Raspberry Pi group's promise of an ARM-based Linux PC for $25. This version costs 10 bucks less than the Model B, and it drops a few features. There's only one USB port, the amount of onboard memory has been halved to 256MB, and the Ethernet chip is gone. These changes cut the power consumption by more than 50%, an apparently important consideration for some users. The Raspberry Pi folks say there's demand for the Model A among roboticists, people doing industrial control and automation work, and even consumers building uber-cheap media systems.
I've been tempted to build a basic media box using a Raspberry Pi board combined with XBMC, which has been ported to the platform. The Model A's lack of Ethernet support definitely limits the options for streaming content, though. The Model B is still incredibly inexpensive considering the capabilities, and at 3.5W, its power rating is still pretty modest.