Slack is a team communication and collaboration platform by Slack Technologies for Web, Windows, Linux and MacOS. Using slack you can do create chat rooms, private groups, and direct messaging, voice and video calls, file sharing and more. It comes in free and paid plans. The free service comes with 10,000 of your team’s most recent messages, 10 integrations with other apps like Google Drive, Office 365 and many more and free 1:1 voice and video calls between teammates. You can check the complete pricing details here.