These weekly recommendations will always be about apps and services that one or more of us in the AlternativeTo love and use. Slack is the latest product we started using as a team and it has really changed the way we work together.
Slack is a powerful, fast and beautiful platform for team communication. They claim to be on a mission to kill email between team members and they sure have succeeded with the AlternativeTo team. You can create channels for any project, team or topic that you need to keep a separate discussion for. At first sight we felt some vibes from the years we spent on IRC back in the days, and we mean that in a very good way.
Besides discussions, private messages, inline images and videos, Slack also integrates with a large number of external services like Dropbox, Google Docs, Github, Pingdom, IFTTT and more. They also have an awesome API that lets you integrate Slack with your own systems. Apps are available for iOS, Android and OS X, with Windows and Windows Phone versions on the way. You may not know that you need this tool yet, but trust us, you do!