XMPP URIs
Contrary to what 
stpeter thinks,
it is possible to hook up external handlers in
Firefox/Mozilla/Galeon/... for arbitrary URI schemes, including the
xmpp
URI scheme.
For Firefox and Mozilla, you simply have to add two keys to
mozilla's configuration. This can be done via
prefs.js or via the about:config page inside your browser. To
hook up a handler for the xmpp URI scheme, you need
a boolean key
network.protocol-handler.external.xmpp set to
true, and a string key
network.protocol-handler.app.xmpp that points to the
program that should handle the URI. The whole URI will be given to that
program as its first argument. It might be that you have to restart
your browser, but after that, links like xmpp:ralphm@ik.nu should fire up the
external program.
For GNOME apps like Galeon, you need to add a few keys to the gconf registry using gconftool-2:
/desktop/gnome/url-handlers/xmpp/command of type string that points to the command. For example xmpp.sh "%s", where
%swill be substituted with the actual URI./desktop/gnome/url-handlers/xmpp/enabled of type boolean, set to
true/desktop/gnome/url-handlers/xmpp/needs_terminal of type boolean, set to
false