The following web sites are the primary sources for the UML standard:
http://www.omg.org and http://www.omg.org/uml
The Object Management Group (OMG)
http://www.rational.com and http://www.rational.com/uml
The following web sites offer other worthwhile information regarding the OCL, the next major revision of the UML, and a virtual community:
http://www.klasse.nl/ocl/indexl
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 Precise UML (pUML) Group.
The following web sites offer other worthwhile information and links:
http://home.earthlink.net/~salhir
Sinan Si Alhir's personal web site.
The Cetus Team offers significant information regarding UML tools, methodologies, processes, and much more.