Open source graph visualization software. Takes descriptions of graphs in a simple text language, makes diagrams formatted as images, SVG for web, PS for PDF, GXL (XML dialect), and more.
Exemplary sites covered here include: WikiViz, FreeMind, Visualizious, Tree Radial Balloon Layout, Comment Flow, OneWord, Del.icio.us Network Explorer, Bubbl.us, ClusterBall, and data visualization of a social network.