Here's a neat database implementation. Have a song in your head, but don't remember the name? Got a microphone hooked up to that workstation of yours? Then head on over to midomi.com, where they have a database just for you. With a very minimal segment of humming or singing (no Frank Sinatra voice required… you can be as awful as those idiots on The Singing Bee), the database should be able to identify your song (and then try to sell it to you).
I haven't tried it (a bit embarrassing on the train), but my head is swimming with ideas about how difficult this task could be to pull off. I'm also curious what technology they're using on the back end… the site is running Apache/2.2.3 Fedora on Linux, so they are almost certainly using MySQL (or maybe DB2/Oracle) and not SQL Server. Not that SQL Server couldn't handle the task, of course, but it's hard for a startup to argue with free.