I've used remote tracker and its one of the better ones (and free). There are other ones out there as well.
One of the newer players is our very own No2chem with his nueGPSSecurity. He is up to version 1.2 (linked below). He should have version 1.3 out any day now.
http://www.nuerom.com/BlogEngine/pos...rity-v112.aspx