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.
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.