What is the best flowchart/graph Open Source Software?

GraphViz http://www.graphviz.org/ is what you want.

It's the standard graph visualization tool for computer scientists, and you can make it do all sorts of tricks from fancy colored graphs with typeset labels to automatically generated call graphs of very large systems with thousands of functions.

It's free and cross-platform. And it uses a simple declarative language to generate graphs, that's easy to create automatically.

If you drop the open source requirement you can get aiSee( http://www.aisee.com/ ) for non-commercial use for the popular platforms. I like this better than graphviz.

xmind's pretty good.

Just going thru the site ... Is it similar to freemind?

never actually used freemind myself but yes, it would be considered a freemind alternative.