iOS/Android: When you're the only one paying attention to your workout, it's easy to slip and let your habits drop. Runkeeper aims to fix this by letting you create groups to help you and your friends challenge each other. In the new app, you can invite your friends to create a running group and add a challenge for everyone. You can choose challenges for things like total distance over a month or runs per week. Everyone in the group will get notifications when you complete runs, and there's a built-in chat to keep groups communicating.
Runkeeper [iTunes App Store]