File photo of a Fitbit Inc. wearable fitness device © Kevin Thrash, Bloomberg via Getty Images

It has been seven years since Nike introduced the workout world to Nike+iPod, a fitness app that married the iPod with a pair of sensor-equipped running shoes to track speed and distance.

Since then, mobile devices and apps have become everybody's favorite training partners.

Today, an estimated one in five U.S. smartphone owners has downloaded at least one app to track or manage their health, according to a November 2012 study conducted by the Pew Internet & American Life Project. The most popular apps track exercise, diet and weight.

Now that the weather is right for outdoor workouts just about everywhere, here is a smattering of low- or no-cost apps recommended by people who use them:

MyFitnessPal: This app is a combination food diary, weight and exercise tracker and calorie counter. It includes a personalized diet profile that users can customize and supports Atkins, South Beach, Zone and other popular diets. MyFitnessPal users can share tips in discussion forums and pair their results with other fitness apps such as Fitbit (see below), iHealth and Tictrac.

Price: Free for Apple iOS, Android, BlackBerry and Windows devices

Fitbit: The Fitbit app works with a wrist band, step counter and thumb-sized activity monitor to track how many calories you burn during workouts, what you eat and even how well you're sleeping. Fitbit devices use a wireless Bluetooth connection to sync with iPhones, iPads and Bluetooth-enabled Android devices to show data on an activity dashboard. "(When) I work out, run or simply have a really active day running after my kids, the Fitbit platform automatically increases my calorie allowance in MyFitnessPal and I treat myself to something sweet," writes Celia Brown on a blog published by software giant SAP.

Price: Free for iOS and some Android devices; Fitbit activity monitoring devices are $59.99 and up

myWOD: Since T.A. Barnhart started doing CrossFit training in January 2012, the 56-year-old Portland, Ore., man has been diligent about working out, watching what he eats and using an app to monitor his progress. (WOD stands for workout of the day.) CrossFit is a trademarked daily regime of weight training, conditioning and cardio workouts that has gained a major following in the past decade. Barnhart calls myWOD the app for serious CrossFit enthusiasts (though it's not officially affiliated with CrossFit, Inc.). Barnhart uses it to browse suggested workouts and to track weightlifting, as well as a timer. He's prepping for the Oct. 6 Portland Marathon, and he downloads data from runs to a spreadsheet to monitor his training. myWOD also syncs with Dropbox, the online file storage service, to store data. "Tech support is awesome, and the improvements they've delivered since I first installed it a little over a year ago have been phenomenal," Barnhart says.

Price: $1.99, for Apple iOS or Google Android devices

RunKeeper: Don't let the name fool you. Along with runs, the app also tracks walks, bike rides, hikes, swims and other activities. More than 20 million people have downloaded it, according to developer FitnessKeeper. Marijke Vroomen Durning, a Montreal nurse and health writer, uses it on the elliptical machine at her gym. "It's very intuitive, and you can set it to remind you when you hit certain spots, like every five minutes," Durning says.

Price: Free, for Apple iOS or Google Android devices; an upgrade that includes GPS tracking and links to Facebook is $5 a month or $20 a year

Nike+FuelBand: The sports apparel and shoe giant is still a fitness app leader, with a slew of offerings. Some, such as Nike+FuelBand, work with the company's wearable fitness monitoring devices. Other apps stand alone, including Nike Training Club, a women's personal trainer app with 100 pre-programmed sessions, including video workouts from pro athletes such as Serena Williams, Lakey Peterson, Carmelita Jeter and Gabby Douglas. "You download the workouts based on the area of the body you want to exercise and can choose different levels and lengths," says Sophia Indja of Sydney.

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Price: Nike+FuelBand app is free and the FuelBand is $149, for iOS devices; Nike Training Club is free, for iOS and Android devices

Charity Miles: One of Texas librarian Katy Manck's favorite apps is Charity Miles, which lets people raise money for nonprofits while they work out. Users must to agree to share their activities on Facebook or Twitter, and they can raise 25 cents a mile for running or walking and 10 cents a mile for biking. The New York startup, which supports 20 charities and has raised more than $350,000 so far, won an innovation award at the 2013 SXSW technology conference in March. "The people who download the app are the ones who are making this successful by spreading the word," Charity Miles founder Gene Gurkoff told Fox Business.

Price: Free, for iPhones and Android devices

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