Clearbit for Slack - Send all signups directly to a Slack channel
Slack Overflow - A programmer's best friend, now in Slack.
Photostash - Automatically save your pictures from Slack to Gdrive
GitCommands - An interactive reference tool for Git commands
Slack It - A bookmarklet to share links in Slack with one click
GitHub + Slack Integration - Full visibility into your GitHub projects right inside Slack