Slack for Unity is the only solution for for using Slack from right inside the Unity Editor. It gives you direct access to the Slack Web App while providing direct integration in a few key areas using the official Slack API.
Official Thread
Features
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• Use the Official Slack Web App from inside the Unity Editor.
• Upload Files and Screenshots directly from the editor to a specific Slack channel.
• View and manage your starred messages in real-time. (great as a todo list!)
• Keep track of all of your reminders and create new reminders for yourself and team members.
• Get real-time Slack notifications inside the Unity Editor - all notifications are archived in case you miss anything.
• Keep track of which team members are available without having to quit working on your game!
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Twitter: @cheapdevotion