PlantUml converts simple text to diagrams.
Ranked in these QuestionsQuestion Ranking
Pro Many, many integrations
For example from Sublime Text. You can learn about integrations here.
Pro Easy create diagrams from text
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 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.