nomnoml is a free and open source tool to generate nice looking UML diagrams from simple text definitions. It's syntax is easy to learn and understand and it knows many UML entity and relationship styles.
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You can override the global styling for the diagram and you can also define custom types for the entities. It is really powerful.
Pro Free and open source
You can extend it, if you find something missing!
Pro Automatically saves
The browser-based tool automatically saves your changes to your localStorage, which is a nice and handy feature.
Pro Fully fledged
It knows many types of UML entities and relationships so you will barely miss anything.
Pro Text format
The tool generates diagrams from a simple text format, which is easy to edit and you can even version control it.
Con Can't connect entities on different levels
The tool can't connect entities on different levels, for example a class and a class inside an other package.