PlantUml is Java-based software that converts simple text to diagrams.
See the The Hitchhiker’s Guide to PlantUML.
Ranked in these QuestionsQuestion Ranking
Pro Many, many integrations
For example from Emacs, Sublime Text and so on. You can learn about integrations here.
Pro Easy create diagrams from text
There is even a Visual Basic Macro to draw UML diagrams in Word.
Pro Free/libre/open source (GPL)
Source code can be browsed here.
Pro Has online editors
Pro sphinx-contrib extension for embedding into docs
Allows you to keep your UML definitions under VCS and include them inside generated documentation with ease.
Con The "plantUML-language" is a bit strange and hard to grasp for new people
Con Slow development due to it being mostly a one-man project
There’s been no recent contribution to the source forge project but from the administrator arnaud_roques. Future contribution might be difficult due to lacking code documentation.