RightGIF - Less noise, better GIFs. A better /giphy for Slack.
Watch2Gether - Watch2Gether is an app that allows multiple people to watch the same content simultaneously and interact with each other while watching.
VixT on Slack - The best video mashup app now on Slack.
Metastream - Watch streaming media with friends
Slack Meme Bot - Spice up your team conversations with memes
StreamParty - StreamParty is a watch communiry, that allows you to watch video streams in sync with the people you care about, on multiple services.