Gephi

Gephi is the leading open-source platform for visualizing and manipulating large graphs.

• Fast: Powered by a built-in OpenGL engine, Gephi is able to push the envelope with very large networks. Visualize networks up to a million elements. All actions (e.g. layout, filter, drag) run in real-time.
• Simple: Easy to install and get started. An UI that is centered around the visualization. Like Photoshop™ for graphs.
• Extensible: Extend Gephi with plug-ins.

Example datasets can be found on our wiki: https://github.com/gephi/gephi/wiki/Datasets

Localization is available in English, French, Spanish, Japanese, Russian, Brazilian Portuguese, Chinese, Czech, German, Romanian, Greek, Hungarian, Korean, Swedish and Ukrainian.