UML's 3 inventors( Grady Booch's methodology for describing a set of objects and their relationships;James Rumbaugh's Object-Modeling Technique (OMT);Ivar Jacobson's approach which includes a use case methodology) came from Ratinal, so I think that Rational is the No.1 UML software products vendors. Other tools like TogetherSoft and Objecteering also used in many companies.