In short:

• One down, http://instantar.org appears to be no more.
• Vuforia has easy entry but eye raising pricing schemes.
• For open access that leaves argon.js (see also http://ael.gatech.edu/lab/research/arweb/argon-an-open-standards-ar-web-browser/)

And perhaps a few other options (https://artoolkit.org/ and http://www.mixare.org/) I will update this post with later.

The software field changes so quickly even Wikipedia has trouble: https://en.wikipedia.org/wiki/List_of_augmented_reality_software

Alternatives

• https://www.aurasma.com/
• https://osvr.github.io/
• https://www.layar.com/

Other references

• https://www.developereconomics.com/top-5-tools-for-augmented-reality-in-mobile-apps from 2015
• http://augmera.com/?p=461 from 2015
• https://thinkmobiles.com/blog/best-ar-sdk-review/ AR app review from 2017, see great chart at bottom of page (e.g EasyAR https://www.easyar.com/ & https://github.com/thinkmobiles/ARInvestigation-EasyARTest)
• https://sourceforge.net/directory/os:mac/?q=augmented+reality
• https://wiki.mq.edu.au/display/ar/Augmented+reality+development+systems (With chart)
• http://sensics.com/open-source-augmented-reality/ (overview)
• Top AR projects http://www.capterra.com/augmented-reality-software/