WheresMyCellPhone.com

If you misplace your cell phone, but know you are in the general vicinity of it, WheresMyCellPhone.com can help you find it. Head over to the site, plug in your number, and it will call your phone. Listen for the ring (or vibration) and zero in on your cell. It will not work repeatedly without a short break. The potential for pranking here is unmistakable, so they give you an option to put your number on their "do not call" list.