What is XMPP?
XMPP(Extensible Messaging and Presence Protocol), also referred to as Jabber, is a decentralized messaging and presence protocol based on XML, as the name implies it is extensible, meaning that anyone can write extensions to the protocol in their own namespaces.
Why should I use XMPP over MSN/Skype/AIM/YIM/...?
The problem with most other messaging protocols lies in its centralization, this means that there is a central server which has full control over the service, meaning it is able to log all your communications, ban you from their service for any reason, etc. This also means that when the central server goes down, the whole network will be unavailable, meaning there is one central point of failure. This model is only beneficial for the person who runs the server, since they have full control over their users and may abuse them at any point in time. XMPP is decentralized, so there is never one server with full control, all servers have control over their own users, of course, but in this model you can choose which server to trust or even run your own, if you don't trust anyone.
I'm convinced, when can I start using this?
Right now! But because in XMPP there is no "main" client or "main" server, you have to choose a client and a server (or host your own), I'll list a few clients, servers and server software below. You can get one of the clients (pick one, if you don't like the one you've picked you can always choose another) and register on one of the listed servers (again, or host your own).
What does XMPP support?
Many, many things, and because it's extensible, more things are being added every day,
XMPP supports things like advanced statuses, multi-user chats, service discovery, legacy protocol gateways, etc...
Here's a list of XEPs (XMPP protocol extensions):
http://xmpp.org/xmpp-protocols/xmpp-extensions/List of desktop clients
Pidgin Site: http://pidgin.imType: Multi-protocol
OS: Linux, Mac OS X, Windows
Source: Open
Jitsi Site: https://jitsi.orgType: Multi-protocol
OS: Linux, Mac OS X, Windows
Source: Open
Gajim Site: http://gajim.orgType: XMPP-only
OS: Linux, (Windows, sort of)
Source: Open
Pandion Site: http://pandion.im/Type: XMPP-only
OS: Windows
Source: Open
Adium Site: http://www.adium.im/Type: Multi-protocol
OS: Mac OS X
Source: Open
And lots of other clients... bigger list at
http://xmpp.org/xmpp-software/clients/.
List of mobile clients
Xabber Site: http://www.xabber.com/Type: XMPP-only
OS: Android
Source: Open
List of Servers
Jit.siJaim.atNeko.imJabber.orgGoogle/Duckduckgo for more XMPP servers, there are MANY more.
List of Server software
http://xmpp.org/xmpp-software/servers/I recommend either
Prosody or
Ejabberd.
Note about lists
None of these lists are complete. If you have anything to add to these lists, just post about it.
MUCs
I'll maintain a list of multi-user chats here, post to get them added.
txug@conference.neko.im
yaypew@conference.neko.im
JIDs
And of course a list of JIDs(identities on the XMPP network) of people, post your JID to get added.
NiXiLL - lrz@neko.im
Port - portify@neko.im
Zack0Wack0 - z0w0@neko.im
(PS. no those are not e-mail addresses, silly forum software)