Researchers Tomoji Takasu and Akio Yasuda of Tokyo University have developed an open source, inexpensive, Real Time Kinematic (RTK) GPS receiver than runs on a beagle board. They've also left instructions on how you can build one yourself.
Unlike traditional GPS, RTK-GPS doesn't measure the delay between its signal and the the signal received by a satellite—instead it measures the shorter wavelengths in the satellite's carrier signal. This can mean accuracies within one centimeter, but there are difficulties with aligning signals with RTK, and the need for multiple receivers makes the technology impractical for navigation. This is why the technology is most widely used in land surveying. If you would like to tinker around with one yourself, full instructions are available on the project page. [Project Page via Make]