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
PlanUML project also allow you to build cool diagrams (Architecture, Sequence, Flow, etc..) using just a plain text format. You can use an online version here. Below an Architecture example with PlanUML definitions.
I have attached a sample below.