A music search engine. Literally, in the same sense that we have text search engines. The user would be presented with a flash keyboard and attempt to input a tune he’s got rattling around in his head. He’d be allowed as many takes as he needed; a sign would advise to wait four seconds then try again, between takes. At the end, the user would select which take is closest and submit it. (There would of course be an optional advanced user interface, for people who know something about music theory.) The performance would be sent in MIDI format, and interpreted. The site would then search its database of midi transcriptions for approximate matches. It would output titles, composer names, years, and albums (if applicable). The user would click on the responses to hear the MIDI files (or perhaps associated digital samples). If the user finds what he’s looking for, there would be unobtrusive yet obvious links to iTunes or Amazon or whatever.

A “name that tune” engine. It’s completely feasible, in theory. The largest hurdle that I can see would be the RIAA.