Nanotechnology is sweet. I mean, in theory at least. The ability to manufacture teeny-tine things out of carbon tubes could bring us space elevators, manufacture on demand, better solar power, and all kinds of amazing things.
Also, it features in The Diamond Age, one of my favorite SF books.
The future, though, is getting closer, courtesy of my home institution, UIUC. Scientists north of Green St. have made a miniture radio out of carbon nanotubes. This is just really neat and makes me smile a bit.