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http://plantuml.com/ allows you to describe your UML diagram in a dot-like language.

Draw.io is also great for working in groups.

You can do an impressive amount of diagramming with the free yED. It also has some automatic layout tools and so on. It's also cross-platform so everybody can use it. The gallery has some impressive examples in it.

https://www.yworks.com/products/yed

PlantUML[1] is pretty decent, especially if you like keeping the contents of the diagram in easily-versioned text.

[1]: http://plantuml.com/

https://www.gliffy.com is rather good.

PlantUML?