Gnublin: The Gnublin board is a small form factor PCB with an embedded Linux  on it. It can be bought on embedded-projects.net. Picture of the Gnublin board:

LED cube: LED cubes are a very popular project , while the sizes go from 3x3x3 to 10x10x10 on with RGB LEDs. At the university we have build a monochrome 5x5x5 LED cube.  

Vacuum cleaner: At the present time there are many autonomous vacuum cleaner. In summer 2012 I have designed and built a vacuum cleaner by myself. The following video shows a short section of it.