A company called SRI International, working as part of the Darpa-funded Open Manufacturing Program, is developing a series of miniature robots that can move around on circuit boards in order to perform various sorts of tasks. There's really no point in attempting to explain what the video illustrates best, though. Have a look for yourself.
The mind boggles at the possibilities for these little doobers. They're already being deployed to build things in the demo there. SRI itself cites a number of potential applications for the micro-bots, including rapid prototyping, the manufacture of hybrid optical-and-electronic circuits, and tissue manufacturing for biotech. Since they track along circuit boards, I want one that moves around my PC's motherboard collecting dust and depositing it out of the back of the case.