Best free flowchart software

yED - is a powerful desktop application that can be used to quickly and effectively generate high-quality diagrams. Create diagrams manually, or import your external data for analysis. Our automatic layout algorithms arrange even large data sets with just the press of a button.

Reference:
http://www.yworks.com/en/products_yed_about.html
http://stackoverflow.com/questions/1181829/best-free-flowchart-software