Whatis - Manage & organize your team's knowledge straight from Slack
Boomerang Respondable - Write perfect emails with Artificial Intelligence
Slack-News - Read news from multiple sources within Slack by just typing
Mailbutler - Mailbutler adds lacking functionality to email clients and increases your productivity
Slack It - A bookmarklet to share links in Slack with one click
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.