Whilst I do not see the point in having three threads about the same thing, I would also be interested in the answer. I have tried a few open-source alternatives to Skype in the past and all of them sucked in one way or another.

Give me one that...
1. Does IM, VoIP*) and screen sharing in one application;
2. Can send and receive files at the same time;
3. Has a Windows**), Linux and Maemo client;
4. Has a UI that does not look like a 14 year old student's homework;
5. Is ideally peer-to-peer, without a central server
...and I will switch in a jiffy and become an evangelist sprrading the good word to everyone I know.

*) where the V stands for both Voice and Video
**) extremely important, no matter how much Estel may not like it.

PS
To Estel: Really. It is not just a matter of telling my relatives to stop calling me on Skype and start using Pidgin. It also means to make their contacts switch in a snowball effect. And for what? Because Skype does not work on my phone? An archaic phone with an obscure OS that no one has heard of? Get real. Use your PC or get a proper phone, would be the obvious answer.