ExtraCore V2 is a $15 Arduino Uno compatible launching a fundraiser on tindie.com.
ExtraCore V1 is sold out. V2 adds an on board power regulator. Tindie only gives you two weeks to raise all the orders you need for a production run. Check out the page now to see more details about the project.
There is lots of neat Arduino stuff on tindie.com already, so you should also check out the site.
(Edit: Fixed links. The Markup added an extra http://.)