I think Fring has a good start. I am using it on my Nokia e61. You load the program (go to their site http://www.fring.com and register, you get SMS, and then download the software).
Then you give Fring your Gtalk, Skype, and other user name and passwords. The software load your friends, and then allow you to chat with them AND TALK with them (cool).
Fring works on both WIFI and 3G – so you can be connected all the time. I had 100% good results in chat; and 80% good results with VOICE (from both WIFI and 3G).
A nice feature allow you to load your regular phone contacts and make Fring calls to them.
Thanks to Yossi Fatael for the link.
Still some work to be done on usability, But a VERY good start.