General information about XMPP
xmpp.org — XSF official website, the prime information source about XMPP.
https://en.wikipedia.org/wiki/XMPP — of course, article on Wikipedia.
https://xmpp.org/extensions — List of XMPP extensions.
Useful resources
IM Observatory — service for testing the security of the Jabber/XMPP network.
Meet the XMPP — great review of technology by IBM.
If you feel we missed some important resource, contact us with your suggestions.