Jabber - a protocol for instant messaging

Jabber is a protocol for instant messaging over the internet. At the same time, it allows setting up of instant messaging at a restricted level in a company's internal network without letting outsiders into the network.
Some of the advantages of employing Jabber technologies are:
- Open -- the Jabber protocols are free, open, public, and easily understandable
- Standard -- the Internet Engineering Task Force (IETF) has formalized the core XML streaming protocols
- Proven - there are tens of thousands of Jabber servers running on the Internet today, and millions of people use Jabber for IM
- Decentralized -- the architecture of the Jabber network is similar to email; as a result, anyone can run their own Jabber server
- Secure -- any Jabber server may be isolated from the public Jabber network
- Extensible -- using the power of XML namespaces, anyone can build custom functionality on top of the core protocols
- Flexible -- Jabber applications beyond IM include network management, content syndication, collaboration tools, file sharing, gaming, and remote systems monitoring
- Diverse -- a wide range of companies and open-source projects use the Jabber protocols to build and deploy real-time applications and services

There are a large number of Jabber clients for different platforms such as Windows, Mac, Linux, PalmOS, WinCE, a lot of them free to use. Some of the most famous ones are Exodus (which allows group chat as well), Pandion, etc. These clients allow archiving of conversations as well.
Some additional extensions make these clients a full-fledged messaging tool, allowing connectivity with MSN, Yahoo and AIM networks as well, thus replacing the Yahoo Messenger, MSN Messenger, ICQ Messenger, AOL Messenger, etc.