Vigilante action against telemarketers! I love this.
When the phone rings and you don't recognize the caller's number on the caller ID, and caller ID says "Name Unavailable", chances are it's a telemarketer. Who Called Us is a site that pools information about those mysterious calls. Start by searching for the phone number that called you: you'll see how many other people they've called and when. Read the comments section, where other victims post what they know about the calling company, and share what you know. You'll even see a graphical map showing where that number has been calling.
There's lots of interesting info on the homepage, including the numbers that call the most and the ones with the most user comments.