Cycle between two tabs with a key stroke.
This extension allows you to quickly switch between the current tab and the last focused tab using a keyboard shortcut or a mouse click. It is configurable in that you can set your own shortcut to use, rather than use the default shortcut (ctrl-b). This extension is open source under a GPLv3 license, so check out the code on GitHub (see the website link below).
Note that this will not work on certain pages, such as the built in extensions page or the Chrome Web Store; this is normal behavior built into Chrome.
Also important is that it is absolutely possible to set a shortcut that overrides/conflicts with default Chrome behaviors. Someday I hope to give a warning for when you set these, but that day is not today.
I have plans for new features, so stay tuned!