Dagre (Directed graph renderer for Javascript) - github.com 
Gephi - wikipedia
yFiles for HTML5 - yworks.com
Viz.js (a hack to put Graphiviz on the web) - github.com
Dracula JavaScript Graph Library - graphdracula.net
Canviz - code.google.com
Why JavaScript? JavaScript is a wonderful language. It is incredibly flexible, dynamic, and expressive. This page is going to attempt to be a collection of the best resources on core JavaScript stuff from the basics to the bleeding edge, with some material original to this wiki in the middle.
Vega is a visualization grammar, a declarative format for creating, saving and sharing interactive visualization designs.