Bike Maps, Cycling Workout, Biking Routes | MapMyRIDE
I've just found it this week, I don't find it the most intuative web site to use but it looks like it's pretty good for keeping track of your workouts. you can upload a route direct from a GPS and it shows all the data from that workout.
If you commute the same route regularly you can register a workout using a previously saved route and it'll give you weekly/monthly totals of milage etc
does anyone use this website already?