SkyTRX Magnet-Mounted GPS Tracker
We've seen mini GPS loggers before, but stalking your girlfriend has never been easier than with this $259 SkyTRX GPS Mini Tracker. It has a magnet mount for easy stickage to the undercarriage of her car, and will record her every move for 100 hours straight.
Plus, after you upload the data onto your computer via USB, you can even map out her comings and goings onto Google Earth—giving you a birds-eye-view of her trip to your friend Larry's apartment.
If you're looking for a less nefarious (read: illegal) use, you can always just use this as a vacation trip logger on a cross country drive, giving your extended family a way to keep up with you virtually.