NMEA 0183 is a protocol that governs data from marine navigational equipment. The most obvious is GPS, but there are a lot of possibilities like sonar, a gyrocompass, and an autopilot.

[stok] built this GPS receiver from the ground up. The GPS module he got off of eBay dumps NMEA 0183 data. He combined that with an old Nokia LCD and a PIC microcontroller. I like this project ...