RightGIF - Less noise, better GIFs. A better /giphy for Slack.
Slackbox - Spotify playlist collaboration through Slack
Slack Meme Bot - Spice up your team conversations with memes
Yarn for Slack - Drop TV and movie clips into your Slack chats
Remixatron - Infinite Jukebox on the command line in Python.
VixT on Slack - The best video mashup app now on Slack.