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
An online app to create sequence diagram that I use is websequencediagram. It's very useful for simple diagrams.