Robin Good looks at a number of tools for real time collaboration. He looks at a number of different tasks & tools – in a presentation (linked to at end of page). He defines “GrassRoots” as low cost (inc. cost effective, open source etc.), easy to set up, cross platform, etc., He does look at some of the interoperable IM tools, though, to be honest, I’m not sure about all of them, as things like Trillian suddenly stop being compatible with MSN, when MSN change, etc.
He’s got a range of VoIP though, besides just Skype.
I’m not going to repeat the whole presentation – watch it yourself!