This is a collection of links to libraries devoted to the visualization of networked and relational data.
Karlsruhe Graph Drawing - An open source library. So far, it contains the algorithm MulMent which is an iterative local optimizer.
A graph editor and a C++ algorithm library essentially concerned with planar graphs. The editor is particularly intended for graph theoretical research.
Java Universal Network/Graph Framework. Java language. Good support for Swing and interaction.
Support Pajek and GraphML formats
OGDF is a self-contained C++ class library for the automatic layout of diagrams.
Python library with data structures for graphs, digraphs, and multigraphs.
Many standard graph algorithms. Network structure and analysis measures. Generators for classic graphs, random graphs, and synthetic networks.
Tulip aims to provide the developer with a complete C++ library, supporting the design of interactive information visualization applications for relational data that can be tailored to the problems he or she is addressing.
Open source graph visualization software.
A Java and .NET library offering a complete Software Development Kit.
A Java, open-source, and free interactive graph visualization and exploration platform. Runs on Windows, Linux and Mac OS X.