some tools for diagrams in software documentation
- AsciiFlow Editor online web-editor, limited shapes
- Ditaa Generator Java
- Ascidia Generator Python
yUML (ruby on rails) can be hosted yourself, for 49 USD
- Sequence Diagram - js-sequence-diagrams JavaScript, browser, SVG
- Sequence Diagram - flowchart.js JavaScript, browser, SVG
- KangaModeling, dot-like, web-api-renderer, .NET
yED is great. The ascii-based ones, I would only ever use for very simple diagrams. That said, text-/relationship-based ones like PlantUML and Graphviz can be extremely useful since you're not so much drawing as "describing". And the crazy stuff TikZ can do knows no limits.