In honor of the Maker Faire going on this weekend in nearby San Mateo, here's a DIY project to track someone from an ordinary cellphone. Using the $100 kit from Mologogo and a $6 a month data plan, a java enabled phone will upload its GPS coordinates to the Mologogo server every few minutes. You can then view the up to 100 of the last reported spots the victim has been on a google map. There's a review up from Phillip Torrone detailing how well it performed around Seattle.
Instructions [Pop Sci]
DIY Cell phone tracking [Make]