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Pro Has many developer-friendly integrations
Integrations for Trello, Jenkins, Travis CI, Heroku, Sentry, BitBucket, HuBoard, Logentries, Pagerduty & Sprintly are available.
Pro Unlimited public rooms for free
If you have a small team, or you are a startup that doesn't need more than 25 per private room, you don't need money to pay for Gitter.
Pro Has full Markdown support
Pro Has apps for OSX, iOS and Chrome
Pro Has an IRC bridge
Pro Open source
Pro Has a Windows 8 app
Unofficial app for Gitter.im is currently available.