Tool: BuiltWith
There are many handy add-ons or extensions out there, but one I really like is the BuiltWith one.
With a single click, you get access to the analytics, connected sites, used techniques, and it offers a link to search for employees over on LinkedIn.
You do need to create an account at BuiltWith to use the extension though. The website provides slightly more information, like IP address history and historical relations and tags. But to have this tool one click away when browsing websites, is worth it for me.
Also, do check out the official link here, and grab yourself their handy bookmark!