I've been playing with the softphones that are out there with the carriers I'm currently subscribed to.
All that said, the X-Ten Pro softphone may provide the most functionality of all. While it's not free, I've been trialing it and have been able to configure it for both Free World Dial Up and iConnectHere once I got the set up instructions from the founder of X-Ten.
As far as handsets go, the Clarisys i-750 is about the best there is. Built in speakerphone, USB port and a touch tone dialer makes it so easy to use.
What all this means is staying connected keeps getting easier and less costly. The per minute pricing ranging from free this month with FWD or .2 cents per minute for up to 500 minutes with Vonage. As someone who remembers .13 cent per minute rates in the nineties and higher before that, the cost of telephone service is being driven down to a point of where it virtually costs nothing. Services are where the money will be made in the future.