I’ve been using Google Voice (http://www.google.com/voice) for a month now and I love it. With the service, you are provided a central phone number that allows you to receive your calls on a variety of phones.
In my case, I have a smartphone and a dumbphone. When I’m using the GPS on my smartphone, I don’t want to take calls because it interrupts my data connection (the joys of CDMA). With Google Voice, I can take the calls on the dumbphone and still use the smartphone for data purposes. The callers are none the wiser. If I need to switch phones during an incoming call, it’s as easy as pressing star.
It’s also useful if one travels to areas where cellphone service can be spotty. I, for example, am spending a few days in New Hampshire. While cell service is spotty, there is a landline phone where I’m staying. With Google Voice, I can add temporary phones for just this situation. By doing so, I can still be reached by phone without having to give out a different number or worry about checking various voicemail systems.
Invites are going out now with fury, so if you’re interested, add your name to the list at https://services.google.com/fb/forms/googlevoiceinvite/.