Great post! We use Slack ourselves, and have experimented with many of the other tools on this list (though we've gone with alternatives like Asana instead of Trello).

Another thing you could add to this list is Pexels.com. We use it for our photos on social and on our blog as it rounds up free to use images from other sites such as unsplash.

In addition to Pexels, we do make use of a few other free tools as well including:

• Headline Emotional Value Checker
• Email Subject Line Checker
• Moz SEO Title Tool
• SEOQuake
• Keywords Everywhere

We went ahead and followed, upvoted and resteemed. Looking forward to more of your great content. :)