cnx.org was the URL used when the organization was called Connexions. It was re-branded a few years ago to reflect its growth and expanded missions. Connexions became OpenStax CNX, within the larger OpenStax organization.

They are all a part of Rice University and based out of Houston, and have been pursuing the goal of providing open education resources since the 1990s.

They're also open source and active on GitHub: https://github.com/Connexions https://github.com/openstax

Source/Disclaimer: I'm a former employee and developer for Connexions and later OpenStax CNX.