And then there were two - VOIP gizmos...

Gizmo, another computer based VOIP implementation is available. It looks like it's very capable competitor to Skype (I haven't tried it). One thing it does allow (and I haven't checked if Skype can do this) is call into other VOIP networks. The question then is - how could you use this service to call someone with Shaw Digital Phone or Vonage (these are not listed in their list of networks). Edit: Business 2.0 has a write up on the app.