Today I learned about websequencediagrams.com which is a handy tool for whipping up quick diagrams showing messaging between systems. I recommend it!
I’ve been using it to create some diagrams of how some parts of Sony Ci (cimediacloud.com) work on the backend for my fellow devs. Ci is an impressively scalable distributed system that performs all of its long-running media actions (like scanning, transcoding, thumbnails, etc) asynchronously via fair-queued worker processes. Since some of the jobs depend on other jobs, so it makes for an interesting data flow, so some visualizations help a lot.