The Open Group is a industry consortium sponsored by IBM, Sun Microsystems, Hitachi, Hewlett-Packard and Fujitsu for forming de facto-standards in the field of software engineering, in particular APIs.
It was previously known as the Open Software Foundation and the X/Open Company.
The group is most known for their publication of the Single UNIX Specification paper - which (in the eyes of many OS developers) is quickly superseding the POSIX standards. In addition, they are the owners of the UNIX trademark. The Open Group provides conformance testing, certifications and white papers usually concerning Unix operating systems. Many of their papers are members-only.
The Open Group or its earlier incarnations has invented and standardized, among other things: