Mermaid is a nice javascript library to generate sequence diagrams, flowcharts and graphs from markdown-like-syntax. It is based on d3.js.
It also has a command line tool, but there is currently no option to style the output with that tool.
I wanted to use it internally for personal projects along with some CSS styling so I threw together a small webpage using angular that generates mermaid diagrams from it’s syntax. The webapp is located here (source)
Right now, I typically type out the syntax and just take a screenshot with ⌘+Shift+4 to embed it in my documents - though I hope this can be improved with proper sharing features in the future.
UPDATE: The webapp has been merged into the official mermaid repository and is located here and several major improvements have been made since!.