As many people have discovered already jabber is the most promising instant messaging technology around. Jabber has a wonderfull architecture which is somewhat similar to email but centered on xml.
A mail message typicaly needs to be passed through an SMTP server to a POP3 or IMAP server where the intended recipient may collect it. Being an IM system jabber messages are pushed to the intended recipient if she is online similar to the activity of a mail relay agent. If the user if offline the message is stored for later delivery. Now jabber acts somewhat like a pop3 server does.
jabber addresses are similar to the email addresses as well. For example the auther's address is firstname.lastname@example.org. If your otganization has a jabber server and and a mail server you might even end up with identical email and jabber addresses :-)