As DanC mentioned, we’ve been playing with configuring Firefox in Gnome to use
tel: URIs with our Vonage service; in short, this means you can click on a link à la tel:+1.555.123.4567 and get your Vonage phone calls the said the number. In practice, your phone starts ringing, and once you pick it up, it dials the said number.
Most of the actual work is done by Click 2 Call which offers a Web interface to your Vonage line. But getting integrated nicely in your browsing/desktop experience is fun.
So, how do you get a new URI scheme to be handled by Firefox or Gnome?