AOPA GPS Course
The Aircraft Owners and Pilots Association (AOPA) has released an online course covering the operation of GPS receivers in aircraft. The course is designed for pilots using GPS receivers for VFR (Visual Flight Rules) navigation.
Although designed for pilots, there is an interesting section covering the history of the GPS navigation system. There is also lots of valuable information covering how the GPS system currently works as well as some of the more advanced GPS technology such as WAAS.
You can view the course here.