Here's a site that's incredibly useful.
Do you ever get those repeated calls where you can't determine who's on the other line from the Caller-ID; the ones that say "OUT OF AREA" or "UNLISTED CALLER"...? Did you ever wonder who or what is repeatedly trying to contact you?
This site does a pretty good job of telling you who is on the other end of the line, and what they want from you.
Just enter the number (from Caller ID - you need Caller ID for this), and 800Notes.com will list any user-submitted experiences with it. For me, it correctly identified a persistent Verizon sales call, and a B.S. collection agency scam.