Earlier this year, Trello introduced premium third-party integrations called power-ups with the likes of GitHub, Slack and Evernote. Today, those power-ups are not available to free users of the service.
Power-ups add a ton of new integrations to Trello, like SurveyMonkey integration that allows you to attach surveys to a card, Giphy support for animated GIFs, or Evernote integration to automatically update notes in Evernote when they're altered in Trello. There is a catch though. Free users can only have one power-up active a time on each board. If you want more, you'll need to update to business class. Still, it's a nice feature and one power-up should be plenty for most people. You can browse through available power-ups on your Trello boards right now, or head over to Trello's blog post for more detailed information.
Announcing Power-Ups for All [Trello]