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Review of Violet UML Editor powered by the Slant community.
Maybe not the best, but easy to learn and use cross platform Java application. Features static (class and object) and dynamic (use case, state, activity and sequence) UML diagrams.
SpecsUpdate
PlatformsWindows, Linux, Mac
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Pros
Pro Simple
Pro Cross platform
Cons
Con Limited
Limited functionality, if more functionality is needed the TopCoder editor might be a better choice.