I recently got Vonage service for my business and I have to say I'm impressed. Vonage has made the technology behind VoIP completely transparent. I plug in my router, plug in my phone and everything works. As more people move to VoIp we will begin to see a convergence between the telephone and PC. Vonage already sells a "soft-pc" version of their software and services like Skype will continue to take off like wildfire.
Technology seems to go in cycles. Phones - > Cell Phones - > VoIP Phone - > VoIP Cell Phones. When there is a critical mass of people who are connected via broadband the middleman will be cut out entirely. The next cycle is VoIP-Peer. Instead of connecting to a Vonage or Skype server you could make a phone call directly to someone else (peer-peer). Never pay for a monthly service, never pay for long distance. A company like Linksys could make a killing on the hardware for these types of devices. The problem is making it easy enough for the masses.
I imagine such a service where you can place a phone call to a persons email address and the recipients phone will start ringing. Such a service would also give a company the opportunity to build a true dial-tone 2.0. A new way of thinking about the phone and how you connect to services and people. More on this later.