Adeona: Free open source laptop tracking
Just found this little gem in Schneier's Cryptogram news letter: Adeona. It's a free, open source, laptop tracker. Just tried it out on a laptop, and it works like a charm.
You install the client on a laptop (or on a desktop, of course), and the recovery tools on a computer you want to use to track the laptop; the recovery tool gives you the internal and (more importantly) the external IP of the laptop, and tries to pinpoint some local routers. In my (brief) test, it pinpointed the laptop's internal and external IP.