I didn't start off keeping track of my runs. I probably should have. It's great to be able to look back and see how far you've come.
When I first started trying to lose weight, I used a service from my health insurance to log food/exercise. I earned points and an awesome gift card during that first push for weight loss. I've used that site off and and on since then, earning more rewards.
I started using DailyMile last year to keep track of my mileage. I reached over 800 miles last year, surpassing my goal of 1100k in 2011. I don't think I'm going to reach that this year, but I still log my miles on DailyMile.
A friend convinced me to start using MyFitnessPal. I love the convenience of the app on my phone and iPad, as well as the website. I also love being able to scan a barcode. For now, I'm sticking with it, although I may double log to earn more points. Can't pass up free stuff, can I?