Smart Development Environment (SDE) is UML modeling plug-in seamlessly integrated with most of the leading IDEs in the market.

Defines UML extensions for business modeling, by Rational. [PDF]

Defines UML in terms of its abstract syntax presented in class diagrams, formal well-formedness rules expressed in OCL, and semantics expressed in text, by Rational. [PDF]

An introduction to the UML that provides a road map to the other documents, by Rational. [PDF]

UML 1.4 Diagrams as quick reference in PDF Format.

A complete reference which can be downloaded for off-line use, by Zvon.