Thank you for your interest in XMPP security!
In the past, the XMPP Intermediate Certification Authority offered free certificates to administrators of XMPP servers. That service, offered at xmpp.net jointly with StartCom for several years, ceased operations in late 2009. However, you can still obtain free XMPP certificates directly from StartCom by visiting http://www.startssl.com/.
Naturally, server administrators can obtain certificates from other certification authorities as well.
If you have any questions about the XMPP ICA or certificates in general, please post to the firstname.lastname@example.org list (visit http://mail.jabber.org/mailman/listinfo/operators to subscribe).