xmpp is a chat protocal that works similar for the user like MSN or AOL
I do not have much experience with it myself but I wanted to use it so now I can!
Whilst registration is free, you will need to abide by the ToS
Terms of ServiceI will try to answer any questions here
FaQAlthough I would recommend checking XMPP.org if you want a more technical answer to any of your queries.