Are there any good VOIP or skype plugins for address book applications so I can just 'click to call' an existing contact? I would like to kick off a call from my address book directly, and potentially integrate that with my VOIP client too. Any ideas?

I realize that skype has a much larger audience, but how do the others stack up? Programmability, integrability, etc.? I have 2 goals: Integratability so I can add voice capability into existing apps. Second, standardize around some solution for our team. Skype seems to be better for getting the team to have voice collaboration. But the integrability is tough. Any ideas about Gizmo Project or some other tool?