Like ARToolKit, NyARToolKit provides both marker-based and Natural Feature Tracking. However, the software has been optimized for ease of portability across different programming languages. If you want to develop an AR application that can run on different platforms and operating systems then NyARToolKit is an ideal choice.
Some of the key features of NyARToolKit include:
NyARToolKit Professional (with Natural Feature Tracking) is available under commercial license only. However, the original open source port has been upgraded to version 4.x of ARToolKit and can be freely downloaded and used under the GPLv2 license from: