For simple diagrams, I've used Pencil (http://pencil.evolus.vn/). I don't know if I'd really want to build anything complex in it though.
For intricate depictions, Inkscape is infinitely more flexible but is a little slow to use for quick diagrams.
Graphviz {and Dot}.
UPDATE: I ended up just using Google Docs. Looks pretty good!
Not specifically for Mac, but I've used the free plan for this Chrome app and it works quite well: https://chrome.google.com/webstore/detail/lucidchart-diagram...