yWorks yFiles for HTML is a powerful library. I have been particularly impressed by its automatic layout features and its capacity to untangle a graph automatically.
The main layouts available are:
- Hierachical layout
- Organic layout
- Orthogonal layout
- Tree layout
- Circular layout
- Balloon layout
- Radial layout
- Series parallel layout
The usage is rather straightforward. Use the page template provided and then write the graph instructions within the script element.
These were probably some of the more popular open source visualization libraries out there.