Virtual Theremin. Occasionally I get these crazy ideas in my head and I can only hope that they will eventually go away. This time, I'm thinking about building my own theremin (even though I know almost zilch about electronics). I've even actually started looking for schematics online. I blame the National Geographic for getting me unnecessarily curious.
Bilingual conversations. I notice "codeswitching" (Kottke's term, not mine) a lot in science with people who don't have English as a first language and especially if they are around a colleague who speaks the same first language. They use their native language with the usual words but sprinkle it with English--especially if it's a technical term. I do it a little bit, especially if there's no English equivalent, but my parents do it a lot more for a different reason--they're not quite sure what the English translation is.
Font Browser. Somewhat useful, although I will have to say I can preview just as well on a word processor or a graphics program.