The Xiaomi Mi Band activity tracker is perhaps the most affordable mass market fitness tracker. The Mi Band can track steps and sleep with it's built-in accelerometer, but also functions as a smartband to alert you to incoming calls, wake you up with its smart vibration alarm, or even unlock your phone (requires requires smartphones with MIUI or Android 5.0 and above). The Mi Band is also compatible with Android and iPhone operating systems. It comes in a variety of band colors and LED lights.