I personally use https://draw.io

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

PlantUML?

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

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

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

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