No more proprietary IM services! See Introduction
You can choose from list 1 and list 2 of public XMPP Servers or set up your own. For me an account on http://jabber.phcn.de/ works fine.
As of 2009-12 / Slack13.0 for me Kopete worked better than Pidgin. Particularly the setup of a different transport like ICQ was easy to set up.
A good web based XMPP client is Jwchat. It can be downloaded or used directly.
More clients can be found at xmpp.org or at the List_of_XMPP_client_software. Empathy seems to support video and may be worth a try.