dailymile, and it's a really great site whether you're a cyclist, runner, walker or swimmer. I have decided to use this to help motivate me as I try to resume my training regime.
What sets dailymile apart from other such sites, is it's social networking aspect. After entering your data from your training, you get the option to share it, with comments, on either Twitter or Facebook, or both. You can also add friends from within dailymile, and track your mileage against their's.
Thanks for reading!