Plant UML – Open-source UML Tool
These submissions/threads have been cropping up a bit lately. But I have yet to seen anyone comment on pikchr, https://pikchr.org/  (NOTE: doesn't have as close of a relation to UML as Plant UML)After what felt like endless googling it is what I decided to spend some time with, haven't had time to do much yet so can't say how it performs for me but the idea and execution really resonates with me.