What: iHeartRate Monitor
When To Use: You should always be cognizant of how hard you're working out. Use this to monitor your resting heart rate, anaerobic rate, and even figure out your Body Mass Index.
Cost: $0.99

What: iMapMy (powered by MapMyFitness)
When To Use: This is a great tool for joggers, cyclists, and power walkers that want to track their training data, from calories burned to pace and total distance. There's no need to pay attention to mile markers because the app uses built-in GPS technology to figure out how much ground you're covering.
Cost: Free* (Ed Note: It appears to be free for now, while the company tries to reach more users.)

What: CalorieTracker by Livestrong.com
When To Use: For those times when you forgo culinary control and are at the mercy of another chef, this application is integrated with technology from the The Daily Plate, so you can figure out the nutritional facts of what you're eating. It will also tell you how many calories you've burned by walking home from your dinner party.
Cost: $2.99