One of the things that I love about GPS data is that they've pretty much decided on a standard -- the NMEA data format. When I first got my Navman GPS for my iPAQ, I thought it was cool. I thought that the included navigation software was cool, and I thought that seeing my exact coordinates was cool. That could have been the end of it, and I would have been happy.
However, most GPS devices dump their data out in a standard CSV format. This makes it very easy for 3rd party software developers to treat a GPS device as a commodity. Rather than dealing specifically with Garman / Navman / etc, they just read the standard. It's great.
It also makes it trivial to write your own apps that interface with a saved data file. I wrote a really small app to overlay a car trip on a map, including red dots where I stopped. Now you can really say, "I'm serious -- look at how bad traffic was!" I've heard of other innovative programs, too, like correlating the timestamp on a picture from a digital camera with the GPS log to give you the coordinates where the picture was taken.
The most useful GPS data is the "RMC" string:
Recommended Minimum Specific GNSS Data (RMC)
$GPRMC,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,<10>,<11>
1) UTC time of position fix, hhmmss.sss format.
2) Status, A = data valid, V = data not valid.
3) Latitude, ddmm.mmmm format.
4) Latitude hemisphere, N or S.
5) Longitude, dddmmm.mmmm format.
6) Longitude hemisphere, E or W.
7) Speed over ground, 0.0 to 1851.8 knots.
8) Course over ground, 000.0 to 359.9 degrees, true.
9) Date, ddmmyy format.
10) Magnetic variation, 000.0 to 180.O.
11) Degrees
12) Checksum.
If you're interested, the data format is here.