The following web sites are the primary sources for the UML standard:
The Object Management Group (OMG)
Rational Software Corporation
The following web sites offer other worthwhile information regarding the OCL, the next major revision of the UML, and a virtual community:
The Object Constraint Language (OCL).
The U2 Partners are focusing on the next major revision of the UML.
The UML Forum is a virtual community concerning the UML.
The following web sites offer other worthwhile information regarding improving the UML:
The 2U Consortium.
The Precise UML (pUML) Group.
The following web sites offer other worthwhile information and links:
Sinan Si Alhir's personal web site.
The Cetus Team offers significant information regarding UML tools, methodologies, processes, and much more.