yEd - Graph Editor. 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. yEd is freely available and runs on all major platforms: Windows, Unix/Linux, and Mac OS X. The latest release is version 3.12.2. Create UML diagrams online in seconds, no special tools needed. Ubigraph: Free dynamic graph visualization software. UbiGraph is a tool for visualizing dynamic graphs.
The basic version is free, and talks to Python, Ruby, PHP, Java, C, C++, C#, Haskell, and OCaml. Download Version alpha-0.2.4. The Diagramming Company. Graphviz.