Vertical tabs has got to be the most useful extension I have installed on my browser. It turns the non-hierarchical mess of tabs that is horizontal tabs, and transforms it into a beautiful little tree, that reflects how I got to any one page. Since I can fold any "branch" of that tree, I can hide whatever tabs I want to keep open but don't want cluttering my tab list.
With the extension I'm using (Sidebery), I also get multiple panels (multiple separate trees), and an easy access to my bookmarks in the same place as my tabs, if I need them.
I honestly think that this extension is in the top 3 things that help me be more efficient in using my PC.