Produce Diagrams from text

This uses the Dot language to generate graphs.

http://graphviz.org

There is a review on the Linux Journal:

http://www.linuxjournal.com/article/7275

If you want to use XML, you can use this tool: http://www.martin-loetzsch.de/DOTML/index.html
Online tool:

http://ashitani.jp/gv/

http://www.ms.lt/ms/projects/toolkinds/index.html