XMPP and the new rise of multi-user chat
XMPP is an open standard for federated multi-user communication, with excellent security and well-established user and development communities. There are a lot of commercial services that provide XMPP support and, despite a couple of high-profile departures from the use of XMPP (for example, Google Hangouts and Facebook Chat, which have deprecated their support for XMPP and are using proprietary, walled garden implementations), XMPP represents a solid foundation for building interoperable, multi-user applications.
One great example is Atlassian HipChat, which is based on XMPP. And while HipChat competitor Slack does not use XMPP behind the scenes, it does provide an XMPP gateway, which allows you to connect your Jabber client to a Slack server.
Compared to the late 1990, when the Jabber protocol was started and desktops were prevalent, today's most popular devices are mobile. The XSF is actively working on ways to improve XMPP support for mobile deployments...