I love exercise. It helps me clear my head, feel good, and burns calories so i can drink more beer. I'm also somewhat competitive. I lever enjoy suffering in the back of a pack, and even when I'm by myself I like to match myself up to personal goals.
Thats why I love MapMyFitness (also the same as: MapMyRide, MapMyRun, MapMyHike, etc.). This great tool easily allows you to map your workouts so you can see how far you went, how much elevation you covered, and even calculates how many calories you burnt. I find the elevation and calorie counter tools to be clutch for my use of map my fitness, as it allows me to compare workouts with ease (e.g., How does a fast run compare to a short ride, but with significant elevation gain?)
MapMyFitness also has a great smartphone app that can track your workout as you go, so you don't have to Manually enter this later. I love this for when i'm mountain biking or running on trails; however, be warned: This tool can drain your phones battery and leave it useless after a few hours.
I love the MapMyFittness Calendar, which shows you your workouts for a given week. A great encouragement to keep logging those workouts. MapMyFittness also has social tools, which motivate you even further by showing you your friends workouts and allowing you to compare yourself to other athletes at your level.
Another cool fitness site is DailyMile, which I think kills MapMyFitness' social features, and has a much cleaner user interface, less cluttered with ads. It's downfall is it doesn't incorporate elevation at all, Which makes it pretty useless out here in CO, where I can do a 10 mile ride with 1,500' of climbing (Note: that's a little bit harder than doing a 10 mile ride on flat ground).
What fitness tools do you like?
Update: Lately i've been using Strava more than anything else to track my workouts. Check it out! it's awesome!