ejabberd is a Jabber/XMPP instant messaging server, licensed under GPLv2 (Free and Open Source), written in Erlang/OTP. Among other features, ejabberd is cross-platform, fault-tolerant, clusterable and modular.

ejabberd Features:
1. Cross-platform: ejabberd runs under Microsoft Windows and Unix derived systems such as Linux, FreeBSD and NetBSD.
2. Distributed: You can run ejabberd on a cluster of machines and all of them will serve the same Jabber domain(s). When you need more capacity you can simply add a new cheap node to your cluster. Accordingly, you do not need to buy an expensive high-end machine to support tens of thousands concurrent users.
3. Fault-tolerant: You can deploy an ejabberd cluster so that all the information required for a properly working service will be replicated permanently on all nodes. This means that if one of the nodes crashes, the others will continue working without disruption. In addition, nodes also can be added or replaced `on the fly'.
4. Administrator Friendly: ejabberd is built on top of the Open Source Erlang. As a result you do not need to install an external database, an external web server, amongst others because everything is already included, and ready to run out of the box. Other administrator benefits include:
* Comprehensive documentation.
* Straightforward installers for Linux, Mac OS X, and Windows.
* Web Administration.
* Shared Roster Groups.
* Command line administration tool.
* Can integrate with existing authentication mechanisms.
* Capability to send announce messages.
5. Internationalized: ejabberd leads in internationalization. Hence it is very well suited in a globalized world. Related features are:
* Translated to 25 languages.
* Support for IDNA2.
6. Open Standards: ejabberd is the rst Open Source Jabber server claiming to fully comply to the XMPP standard.
* Fully XMPP compliant.
* XML-based protocol.
* Many protocols supported3.

Additional Features:
Moreover, ejabberd comes with a wide range of other state-of-the-art features:
1. Modular
2. Load only the modules you want.
3. Extend ejabberd with your own custom modules.

Security
1. SASL and STARTTLS for c2s and s2s connections.
2. STARTTLS and Dialback s2s connections.
3. Web Admin accessible via HTTPS secure access.

Databases
1. Internal database for fast deployment (Mnesia).
2. Native MySQL support.
3. Native PostgreSQL support.
4. ODBC data storage support.
5. Microsoft SQL Server support.

Authentication
1. Internal Authentication.
2. PAM, LDAP and ODBC.
3. External Authentication script.

Others
1. Support for virtual hosting.
2. Compressing XML streams with Stream Compression (XEP-01384).
3. Statistics via Statistics Gathering (XEP-00395).
4. IPv6 support both for c2s and s2s connections.
5. Multi-User Chat6 module with support for clustering and HTML logging.
6. Users Directory based on users vCards.
7. Publish-Subscribe7 component with support for Personal Eventing via Pubsub8.
8. Support for web clients: HTTP Polling9 and HTTP Binding (BOSH)10 services.
9. IRC transport.
10. Component support: interface with networks such as AIM, ICQ and MSN installing special tranports.

The license of this software is Free, you can free download and free use this instant messenger software.