I've been using Mapmytracks.com to record my cycling to and from work for the past year now. And if I were to offer any advice to aspiring cyclists who wish they could ride more, it would be to get a gps and record all of you rides. I found gps to be the best motivational tool available. Mapmytracks.com has multiple ways of analyzing your ride and comparing it to other similar rides in the past. You can figure out where you have gained speed and where you need to work on your riding.
The program can be combined with more serious training tools, which I do not use. However, it is possible to measure cadence and heart rate, and sync this data to your gps data to get a complete picture of your ride and the amount of exertion you are putting into your workout.
At any rate, today is the end of my first year recording my rides. My total mileage on my bike came out to 5059 miles. Not bad for a year of commuting to work on my bicycle. Assuming I drove those miles in our Prius, which gets 45mpg, and $3.50/gallon of gas that's 112 gallons of gas saved, and $433. Nice.