Have you ever looked at your phone while there was an incoming call and wondered, "who is this calling me?" You stare at the number, flipping through your mental Rolodex all the while, trying to place the phone number. You think back to all your friends that may be calling from various area codes across the nation. Then you either resolve to wait until the caller leaves a voicemail, or you do an online search.
What if you could search a database of numbers from telemarketers so you know to avoid them in the future? It sounds a lot like the DoNotCall list that's circulating around many of your email inboxes, in reverse. CallerComplaints has created a site for this very purpose, and it lets you submit numbers from telemarketers, pranksters, debt collectors and political campaigns.
This user-generated database is a self-regulated community that lets you submit numbers that you're complaining about, leave comments, and provide any other useful information of which you're aware. In performing a search, you can find out the basic info for a number, even if there aren't any complaints against it, and see its site stats--how many complaints have been filed for that number, how many times has it been searched, etc.
Of course such vigilante sites such as Caller Complaints and PostaCrime have the potential to be a highly abused system, but as we've seen with wikipedia, the average from all user activity will reduce the margin of error and abuse.