Boarduino is a “breadboardable” Arduino clone created by Ladyada. It has a much smaller form factor and it’s designed to plug right into a solderless breadboard, making your circuit prototyping efforts a breeze. How cool is that?
You use it just like a normal Arduino. You can slap the standard Arduino bootloader on it and run all your normal Arduino sketches. The only major difference is that it doesn’t have the USB hardware on-board. Instead, you’ll need to use a USB-TTL serial cable ($20 at the Maker store). There is a convenient 6-pin header for this purpose.
The whole thing can be purchased as a kit for $17.50 from the Aadafruit store, making this one of the cheapest Arduino kits available. Check her Boarduino page for details on building, testing and using it.