WebGraph by PhilXD

When you click on the plugin, it opens a graph window that shows the currently opened tabs. When you use links to open new pages, it will add them to the graph with a connection from where you opened it from, showing you the connectivity between pages.
There are features like
groups, allowing you to label a group of nodes, and move them all at once,
and Merged nodes which are multiple nodes of the same website merged to one node, which you can also have it do automatically using "Group by domain".