See all of the previous submissions of a URL posted to Reddit and find new communities in the process
Use this information to find previous discussions and different subreddits you would not know about otherwise!
The table that appears under each post links you to the individual thread and the Subreddit for every past occurrence of this URL on Reddit.
You can toggle the feature on and off for the current page using the new tab you'll find on the Reddit tab bar.
Or toggle the feature for individual posts by clicking on the "x other submissions" label below each post title.
Disclaimer: This extension required making a lot of queries against the Reddit API, for this reason your browsing history will be cached along with other users to improve the overall user experience.