https://plantuml.com/ is free and nice. Plays well with Markdown and C4 diagrams.
The only UML-specific diagram type I use is the sequence diagram but that is very useful IMHO.
I could have never maintained the diagrams I used for other developers on my teams or folks on the audit/regulation side of thing without it.
The default styles are really unprofessional looking and I think a lot of folks look the other way once they see that. C4 diagraming with PlantUML is also a breeze for systems diagrams.
Also playing with 11ty and this to replace our home grown tooling based on markdown with embedded dot.