For those who are having trouble remembering the name of a song, Midomi allows you to search by keywords... or by humming or singing into your computer's microphone. It looks like they are in the process of building a database, but it seems like an interesting idea!
Though most songs are still missing, for example I tried playing the song "Mas Que Nada" (Sergio Mendes) through external speakers while recording it using the search feature to find it, so instead of humming or singing I played the actual song. It showed the search results: Justin Timberlake songs etc, nothing even _close_ to that song I wanted. Ok, I know it's cheating, but if it won't even find songs if you play the real one, what will it find if you sing it yourself?? But I'm sure, after some years it'll work out. But, we'll see...