Other
Uml Home Page
The OMG specification for UML is considered the industry standard. This site contains the current UML specification, UML tools, tutorials and reference materials.
Other
Practical Uml: A Hands on Introduction for Developers
A tutorial that provides a quick introduction into the use of UML.
Wikimedia
Wikipedia: Unified Modeling Language (Uml)
A Wikipedia description of the Unified Modeling Language (UML) specification.