It's both... an open protocol, and there are opensourced servers and clients, as well as commercial servers and clients. Think of it as similar to email, but for IM.
Jabber.org is the home of the most complete (unix based) server.
You can see a list of (some of the) public servers running on the web at
http://www.jabberview.com - also
http://www.jabbercentral.com has a fairly complete list of jabber clients.
It may not be as simple as adding a link to the ICQ webpager is - ie some server side scripting might be needed, but we would really appreciate it if you would take the time to let us know what you need. I am happy to help out as much as I can, or to shanghai someone into helping :-) But this is what open sourced projects are all about hey?
Let me know what you need. All the specs are on the
jabber.org site, and there are guys that are good at scripting hanging out on the JDEV list
http://mailman.jabber.org/listinfo/jdev/Being a completely open protocol really means Jabber has some great possibilities for interacting with YaBB that probably haven't been thought of yet. There is nothing (except for time and energy :-) to stop you guys (say) running a Jabber server
jabber.yab.info with scripts to ping off headline format instant messages to people when new posts arrive etc, or allow people to post messages via Jabber clients etc etc.
Zilch.