A FIRST round OF useful UML resources online IS the following:

• UML Standard Specification: Official UML documentation
• UML Resources page FROM the OMG: Collection OF tutorials, articles AND links FROM the OMG
• Best UML cheat sheets AND reference guides
• Practical UML: a hands-ON introduction FOR developers : Quick tutorial covering the basic notation concepts OF the UML.
• Cetus links ON UML: Outdated AND messy but extensive list OF links TO UML-related information
• UML mailing list: Distribution mailing list TO discuss ALL aspects OF UML. Always glad TO help newcomers.
• Precise UML mailing list: Mailing list OF the precise UML GROUP INITIALLY created TO propose a formal semantics FOR the UML LANGUAGE. Messages IN the list ARE mainly related TO formal aspects OF the LANGUAGE, interpretation OF the specifications AND research conferences. Therefore, don’t post novice messages here (but it maybe worth to read through the list archives)
• Conrad Bock’s links: Basically covering the behavioural diagrams WITH emphasis ON ACTION languages TO make UML executable
• Mario Jeckle’s collection of links: on UML. The page is in german but the referenced sources are mostly in English
• UML-forum: News and resources about UML
• LinkedIn UML Lovers Group
• Glossary of UML terms in wikipedia
• Parlez uml : A couple of nice (and free) tutorials about UML. Simple but to the point.
• Powerpoints about UML
• Free UML videos (variable quality)
• Twitter accounts of UML, modeling and model-driven engineering tools
• Free examples of UML certification exams

Do you know other valuable resources? Let us know!