Nike+
Nike+ has always been a great way to keep track of your runs, but until recently, was iPhone only. Nike+ now works with Android phones and iDevices. For runners, the Nike+ system is a well designed way to track your speed, heart rate, and location data. If you aren't a runner, use Nike's fuelband to track all your other activities. Nike+ measures workouts in "fuel," which you can stack up against other Nike+ members.

Strava
The Strava system is targeted mainly at cyclists, though it tracks running workouts as well. Strava works with your iPhone, Android phone, or Garmin GPS device to track routes, speed, elevation, heart rate, and power output. Strava even ranks you against other users. View segments of rides and see how you perform against everyone else on the same segment--nothing is more fun than becoming the king or queen of the mountain!

MapMyRide/MapMyRun
Like the previous sites, MapMyRide and MapMyRun keep track of your speed, distance, and routes, among other data. The best feature of the site is the ability to plot routes, eliminating the need to track your run or ride in real time. The site is especially nice for plotting a bike ride ahead of time, as it will show you elevation data, and ranks how challenging the hills are on your route.

Garmin Connect
A Garmin device is probably the ultimate tool for tracking your workouts. Garmin makes gadgets for running, cycling, swimming, hiking and even golf. Their best models track everything, from basic speed and GPS, to cadence and even swim distance. While Garmin's data is versatile enough that you can export it to most sites (like Strava), Garmin's own Garmin Connect keeps track of all the data their devices record.

Daily Mile
Dailymile is like Twitter for your workouts. Input an activity you've completed in the day, and it appears in your workout stream--as does the activities of your friends. You can input as much information about each workout as you like (mileage, pace, route--they're all optional), and your gym sessions and workout classes, too.

Fitocracy
If playing games keeps you motivated, look no further than Fitocracy. Fitocracy allows you to join challenges, accumulate points, and ultimately level up. If the idea of "leveling up" through your workouts appeals to you, give fitocracy a try. It's a social gaming service for fitness.

RunKeeper
RunKeeper is the less stylized version of Nike+, and is also available for iPhone and Android. It's a clear and easy to use system for tracking your running routes, pace, and distance. RunKeeper also offers some great training programs. Use its "FitnessClasses" feature to create a training guide for your next race, be it a 5k or marathon, or just to make a new weight loss plan.

Endomondo
Endomondo is a social site for tracking your runs, bike rides, and hikes. Like most other sites, Endomondo offers challenges and updates from your friends. Best of all, Endomondo works with pretty much every smartphone under the sun. If you use something besides an Android phone or iPhone, be sure to give Endomondo a try--it works with BlackBerries, Windows Mobile Phones, and even Nokia phones running SymbianOS.