Slack has become the go-to messaging app for start-ups and legacy businesses that want their teams to be able to communicate in a convenient way throughout the workday.The company claims that it's the fastest growing business application ever. Founded in 2014, Slack already has 9 million weekly active users. Built to bring teams together and to cut down on emails, Slack also boasts 50,000 paying companies, including 43 companies from the Fortune 100 list. Companies from AirBnB to Buzzfeed to Oracle are using Slack to help facilitate team and inter-team communication.