RunKeeper‘s humble beginnings as a simple running app managed to attract a massive user base, which now stands at more than 5 million users and counting. Their latest app update featured heart rate monitoring, selective sharing, mobile and web sync. And in the past few months, RunkKeeper has integrated with wi-fi body scales from Withings, heartrate transmitters from Polar and Wahoo Fitness and Garmin watches.

Today, RunKeeper announces its newest integrations with modern fitness devices Fitbit and Zeo, which can both be found in the RunKeeper store.

Integration with these devices is an exciting step forward in our quest to provide the RunKeeper community with the ability to track, measure, and improve all of the key fitness metrics that they care about as part of their core RunKeeper experience.

-CEO Jason Jacobs

Screen shot 2011 03 29 at 4.08.32 PM 220x251 Fitness gets futuristic: RunKeeper integrates with Fitbit and ZeoFitbit tracks all of your motions during the day and at night, as well as total calories burned and consumed and how long and how well you’ve slept. Fitbit will then update your RunKeeper profile automatically with this data so you can see your full day of calories burned, hours slept, and steps tread alongside your training in your FitnessReports. The device, pictured right costs $99.95 and is available in the U.S. only.

Zeo is a sleep device that offers even deeper insight into your sleep patterns. It counts the number of hours you sleep each night, including the hours of REM, deep and light sleep you experience, the number of times you wake up each night and the impact of sleep on your fitness performance. The device is even used by US Triathletes including Champion Jarrod Shoemaker. With RunKeeper, it synchronizes all your data just like the Fitbit. The device, pictured below, retails for $199.

Screen shot 2011 03 29 at 4.27.09 PM Fitness gets futuristic: RunKeeper integrates with Fitbit and ZeoRunKeeper continues to work on making its API public, so that many more devices, apps and websites will be able to integrate with the growing platform, providing RunKeeper users with a holistic view of their health across the different health fitness data types that they care about in one place.

Healthy living has always been important to me. Looking towards the future of fitness and technology, I believe sensors will proliferate so that more and more interesting data will be collected in a frictionless way. Just by doing activities there’s more you can learn about those activities. And when you have that information, you can be more proactive in making changes to improve your performance…In the future, I will literally just slap on my shirt and running shorts and head out the door and not have to do a thing and then have all my data waiting for me when I get back that would be a great future. The same with weight lifting, food I eat, how I sleep at night, walk around, etc. It sounds big brotherish but in the future we will have the capability to track your every move.

-CEO Jason Jacobs

Earlier this year, RunKeeper decided to give its Pro app (usually $9.99) away for free, for life. It still makes money off of its RunKeeper Elite, a subscription web-based service with fitness alerts and real-time workout sharing for $4.99 per month and $19.99 per year.

RunKeeper was included in our list earlier this year, titled The Best of Boston Startups. With summer just around the corner, it’s probably time to get off your computer and start exercising, yeah?