It took a while to figure out
what I was looking for. It's the type of component that lets you make one of those continuously turning control knobs that seem to be popular on modern electronic devices.
I feel an urge to re-learn some electronics and build stuff.
Update: d'oh - a bit more research reminds me I can get something similar by opening up a mechanical mouse. I'm sure I have one lying around.
More update:
here's someone's take on the idea. Doesn't say anything about the software side though.