If you're looking for something to connect to a TV, the MK802 or MK808 is clearly the winner. If you're looking to make a toy, or run some lights, than Pi is the choice. I have the UG802 and it's very small/powerful for all that it does, but XBMC doesn't support HW decoding yet. Raspberry Pi looks cool, but the specs are lousy compared to the alternative. Another obvious choice that no-one mentioned are used Android Phones. I just upgraded my phone and have a Galaxy S phone no longer being used. Still looking for a good project to make use of it.