A simple Safari app extension that adds a button to your toolbar that when clicked, will take you to the latest archived version of the current page on archive.org's WayBack Machine.
Hacked together in a couple of hours, still a WIP and so it may have bugs.
WayBackButton requires access to all tabs in order to function. It uses this access only to read the URL of the current tab; no data is transmitted or recorded, as can be verified in the source code.