I have a home phone (Vonage), a cell phone (Sprint), a cell phone extender/femto-cell (Samsung Airave), a VoIP phone connected to an Asterisk server, Skype, Adium, and a Mac Pro.
I work from home, so much of my time is spent in front of my computer. My kids are home from time to time taking a nap, and my wife is at home fairly often as well, so I wear headphones almost constantly so I can listen to music and other beeps and boops from my random communication applications without bothering others, and at a low enough level where I don't get hearing loss, but I also don't hear the outside world.
Although the poster asked only about one solution, I can see where they are getting at -- Telecom integration.
It would be convenient to be able to answer all of my various incoming communications -- cell, land line, Skype, SIP and VoIP -- on my desktop. It would also be convenient to be able to place calls from my desktop to others via whichever route I choose -- Skype, Jajah, Google Voice, SIP, cell phone, land line -- or to be able to build a little LCR (Least Cost Routing) db that chooses for me, based on criteria I can set. Having access to all of those pieces in a desktop format would enable me to do some cool stuff, and also allow providers I use to add new features via an API to make communications even easier.
As it is now, in theory this is all possible, but no one has come up with an easy way to integrate it all. Sure, there are hacks like Asterisk, but then you have to run an additional server or Virtual Machine, and it isn't for the feint of heart. A desktop app that could do this would be very slick, but there is still the difficulty of integration. Is it a single device which handles your cell and land lines? Can you transfer a call in Skype through your desktop to your home phone line, so you can take a call in the bathroom? Maybe I need to leave and want to continue talking on Jajah but transfer to my cell.
It just doesn't yet exist, and if it does, it is difficult to do.