Monodraw is great. I occasionally use it to document system architecture in code comments and project READMEs etc. [0]

Also handy as a quick way to document CSS styles to help visualise which parts of a layout a given style targets [1]

[0] https://gist.github.com/supertr0n/f006a5f61b60160862ec13024e...

[1] https://gist.github.com/supertr0n/462325ec3f7fbea02780039150...