I believe Google Calendar also asks if you want iCal files to be opened in Google Calendar. I think other sites could register as that protocol handler as well.

Any protocol can request to be registered with the browser. Just a simple line of JavaScript[1]:

window.navigator.registerProtocolHandler(scheme, url, title);

[1] http://www.w3.org/TR/html5/timers.html#custom-handlers

irccloud.com (no affiliation, it's convenient though quite buggy) will open irc: links.