Or you could provide a jabber server that uses your web app's authentication mechanism so users just need to add another IM account to the IM account they are a

Or you could provide a jabber server that uses your web app's authentication mechanism so users just need to add another IM account to the IM account they are already using. Or things like growl could take an jabber server as a configuration option to receive notifications from. Desktop notification is mainly just a reformatting of the UI on technologies that already exist, are in use, and are pervasive.