Clearbit for Slack - Send all signups directly to a Slack channel
Humbler - Find events that support causes you care about
Photostash - Automatically save your pictures from Slack to Gdrive
Event Finda - Find events near you, and all around the world.
Slack It - A bookmarklet to share links in Slack with one click
Rally by Eventbrite - Find the best local events and rally your friends to join