[wainstead]

+1 for plantuml. You can make very useful sequence diagrams with it, in ascii no less.

[_puk]

It would be remiss at this point not to recommend websequencediagrams[0].. been using it for years (and convinced multiple workplaces to shell out for the self hosted version..). It's great :)

0: https://www.websequencediagrams.com/