ARToolKit is the world's most widely used tracking library for Augmented Reality usage.
Transparent and modular open-source code:-
You get full access to the computer vision algorithm so you can easily modify the source code to suit your application. This is freely distributed under an LGPL v3.0 license.
You can easily download and use compiled SDKs for Android, iOS, Mac OS X, Linux, and Windows.
Develop for smart glasses and more:-
ARToolKit offers you optical and stereo see-through support and is integrated with a large number of smart glasses. On top of this, you can easily calibrate it for new devices.
Mobile focus for an easy development process:-
This includes OpenGL ES2.x support, utilities for automatic camera calibration, and good integration with GPS and compass.