The best apps for Team communication (other than email and telephone)

The best apps for Team Communication, which include Slack (and it’s benefits with centralising communication during project work), WhatsApp (a really quick and easy tool for group communication that doesn’t have to be confined for use on just your mobile, as it works especially well on your PC browser), and Google Hangouts (for face to face online communication similar to Skype but with the ability to group chat with up to 10 participants for free).