A couple of days ago I stumbled across the Tree Style Tab extension for Firefox which organizes tabs as a tree based on what tab was used to what other tab. Since putting all that into the normal tab bar of the browser would have been really too much, UI-wise, the tab tree is shown in a sidebar, completely replacing the default tab bar.
What makes that tree of tabs so useful is that you can easily close a tab and all its “children”. It also helps me tracing back where I originally stumbled upon a page… if I’ve kept all the relevant tabs open, that is.
If that’s not enough, TST even offers its own API so that other browser extensions can enhance it.