(ascii to diagram)
Typogram https://code.sgo.to/typograms/#installation
Markdeep https://casual-effects.com/markdeep/
svg bob editors https://ivanceras.github.io/bob-editor/ https://mbarkhau.github.io/asciigrid/
Ditaa https://ditaa.sourceforge.net/
Goat https://github.com/blampe/goat
Protocol https://www.luismg.com/protocol/
(dsl to ascii)
https://diagon.arthursonzogni.com/#Sequence
Also I used to use https://swimlanes.io/ before d2.
https://github.com/lewish/asciiflow
https://github.com/tbanel/uniline/
Or, less ergonomically, the general rectangle-editing commands built into more powerful code editors e.g. Emacs.
With these, simple but fairly pretty box+line+text diagrams can be inlined with your source code comments. This unification may help in the perennial struggle to keep software architecture and source code reality in sync.