Emacs, at least, already has artist-mode built in.

Screen cast showing it off: http://www.cinsk.org/emacs/emacs-artist.html

DrawIt for Vim: http://www.vim.org/scripts/script.php?script_id=40 . I think I started using this in the previous millennium, and if not, not much later. It's awesome. Not all (former) coworkers agree with that sentiment though, to be honest.

Thanks for your suggestions, roel_v and a_e_k! Will play around with DrawIt, looks fun!

Two other tools that render images from ascii are ditaa (http://ditaa.sourceforge.net/) and plantuml (http://plantuml.com/). Those tools, some scripts to render and combine markdown and images, and DrawIt (+ some other vim tools like 'boxes' (not sure if that still has a web page), 'sketch.vim' and 'boxdraw') make for the least painful, versionable documentation system, for me. ('least painful' in the sense that I still need to code way too much to get it all to work, but there isn't anything ready-made and stable enough that I have been able to find the last decade).