Messaging apps have been one of the business mainstays for over a decade. Where Microsoft Messenger/MSN once ruled, there are now heaps of alternatives offering basic chat as well as collaboration tools to share screens, desktops and audio/data/video.
Skype is one of the most common chat, peer-to-peer VOIP telephony and video calling tools. It works on almost every platform and is broadly free to use, but charges for network calls to landlines and mobiles.
For a platform that organises messaging feeds as well as offers file sharing and group chat, HipChat also works on all popular platforms and devices. The $2 p/m paid version adds video chat and screen-sharing.