Quick tab navigation. Jump to last tab (previous tab), bookmark tabs, jump jump!
If you have a lot of tabs open, the ability to bookmark tabs and jump directly to them, as well as to the previously active tab, will save you a lot of time. This extension does that.
I personally make GMail #1, music streaming (beastmode.fm) #2, Netflix #3 and quickly bounce between them and 50 other tabs.
[Updated 1/26/12 to work across multiple windows]