Also, plug computers like FreedomBox: http://en.wikipedia.org/wiki/Plug_computer
And single board microcontrollers like the arduino boards, make controller, basic stamp, etc.: http://en.wikipedia.org/wiki/Single-board_microcontroller
I personally would like to see a build-your-own tablet type of thing that can run android or ubuntu. Of course there already is the nexus 7 though.