This FREE app takes GPS readings at a user-set intervals and stores them online in several formats including raw data, various file forms (gpx and kml included) and graphically using Google maps. Like SPOT, you can also enable "Sharing" and allow others to track your movements online.
The way it works is the app uses the device's GPS to take a reading at a user-definable schedule (mine is set to every 180-seconds) and it then uploads it using the phone's data connection. The phone will buffer 100 readings (they are promising more in upcoming updates) if you are out of data coverage then upload en masse when data coverage is re-established.
The real value of the site (aside from it being free) is the tools for selecting and downloading the tracks. The site stores over 100,000 points so you can keep months of rides, and the ability to select and store segments makes it easy to keep rides and trips in order.
I've been using it for a few weeks now and have become a big fan, I did have to increase the reading interval to 3-minutes (stores 5 hours of "out of range" riding) as I initially lost some big track chunks from being out of coverage areas when it was set to every 5 seconds (only 10 minutes or so of "out of coverage" riding.)
The downloaded tracks come right up in Google Maps, Google Earth, and even Garmin's proprietary mapping program, and not having to stop and make a note of neat road discoveries is a real benefit.
Best of all it is free - just make sure you have an unlimited data plan! One side note, if you have a Blackberry make sure to "switch applications" for it to run in the background, if you simply exit out or hit the return to main screen button the program will shut down. iPhone users will have to keep it open until the upcoming iPhone OS release finally ushers in multi-tasking.