Archive relates to historic or previous versions of a website, even websites which may no longer be online, so archive sites can be really useful when researching online sites
Cached Pages relate to copies of a webpage, usually held by a search engine from the last time it visited the webpage, so can be useful when researching sites which may have recently changed or been taken offline
Archive Sites
Archive Request To Save Sites
Cached Pages
Even though Google has now removed the Cached function from its search results, you can still use the below links or Add-Ons / Extensions to view the Google Cached page of a webpage you are viewing. Or you can add the full URL of the webpage you wish to view the Google Cached version of the end of this URL. Also don't forget that sites like Bing & Yandex still display the Cached / Saved Copy in their search results.
Firefox Add-Ons For Archive & Cached Related Tasks
Chrome Extensions For Archive & Cached Related Tasks
Website Copiers & Downloaders
Website Copiers & Downloaders From Archive.Org
Webpage Date & Time
Not an add-on or extension but we demonstrate this on our course and it is a little useful tool, so if you want to save this feature to your Favorites or Bookmarks right click on Web Page Date and then click on "Add To Favorites" or "Bookmark This Link", or just left click on the Web Page Date link and then drag it to your Favorites bar, to assist in showing when a static webpage was last updated