I've been using Alientube for a while but couldn't find it here on PH while I was setting up a new computer, so I figured I'd throw it up and spread the word!
Essentially, Alientube looks to see if the YouTube video you're watching has been shared anywhere on Reddit. If it finds something (it usually does), it replaces the standard YouTube comments section, which is typically full of spam and vitriol, with Reddit comments from (one of) the most popular submission(s), bringing the diversity and nuance you've likely come to experience in the Reddit community to YouTube.
I highly recommend you give it a shot! It's updated semi-frequently and available for all of the major browsers, including Safari, Chrome, and Firefox.
Nest Thermostat