Audioscrobbler records what you play on your computer, looks at other people who listen to similar, and gives you recommendations. Tie it in with an online radio station, and you’re in techie music heaven.
OK, there are some obvious privacy issues here. Like, the plugin is designed to be transparant (a good thing), but you’ll forget it’s there when you’re playing stuff you might not want the world to know about.
I’m experimenting with it. Sign-up took 1 minute, installing the plugin took 1 minute. So far, so clever. Exactly the sort of thing you’d hope the web would make easy.