Shortcut to the last visited tabs.
Click the icon to select the last visited tab, click again while the counter is on to select the next-to-last one, etc...
When the counter turns off, clicking the icon sends you directly to the first tab you were on before going back in time.
You can customize the delay before the counter turns off: right-click the icon and choose 'options'.
You can assign a keyboard shortcut to Backxtab: chrome://extensions/shortcuts