Check out SJPhone in this thread over here:
http://forum.ppcgeeks.com/showthread.php?t=126802
Right now I'm using Google Voice + Gizmo + Fring.
The fring call quality seems to be fairly poor and I'd like a better solution. My problem with SJPhone is that I have been able to get it to work with the gizmo sip proxy and my account, etc. I have not yet been able to answer calls with it. I keep getting an ACK (acknowledgement) error. otherwise SJPhone seems to be a pretty nice SIP client.