Bitrix24 is the world's only free unified communications software available both in the cloud and on-premise. And you get everything - enterprise social network, company instant messenger, telephony (PBX), video chat and video conferencing, email server, file sharing and project management. If something is used for communication and collaboration, chances are it's one of the 35+ free unified communication tools available inside Bitrix24. API, content management and open source code are all available.
* Please note that the information may have changed since the publication. For current prices and features please visit Bitrix24 pricing page.