Make, as usual, has an excellent round-up of open source hardware kits based our own favorite, the Arduino. The list includes all the add-ons, like the Ethernet and Motor shields, and devices like the Seeduino, a more powerful Arduino clone.
My buddy Paul has a few of these and is building stuff as we speak. I don't really have the patience for this kind of low level electronics, but I'd love to hear about some projects you guys have cooked up. Anyone?