TinEye is a reverse image search engine-- ie you upload a picture from your computer and it'll look for sites that use that picture. Seems to work pretty well, though sadly it was unable to tell me where the above picture came from-- it's been sitting orphaned on my PC for ages. Their Cool Searches page shows some examples of what the site is capable of in terms of image recognition-- impressive stuff. Similar to this, Google has a Similar Images search function, which seems to work pretty well but doesn't seem to support searching uploaded images so far.
Hmm, I wonder what it'd take to make a content search for music-- I'm not sure how input to the search would work, but it'd be an interesting project just to study feature/melody extraction from mp3s. Most music has some sort of regular structure: could you automatically find the hook or the chorus of a pop song? Maybe make a filter that converts complex orchestral sound to pure tones, or even generates sheet music from sound files? Time to do some digging.