Basically just remote desktop limited to one program. There would be a system similar to online game matchmaking to connect users with one another easily.
I can think of two ways that this could be implemented - either have the applications themselves have matchmaking systems, or have it be a general connection program wherein you would find a peer and then select a program that you both have.
It would be especially useful in creative applications like Photoshop or FLStudio (music synth/sequencer program) as you could have direct collaboration remotely.
There would also have support for more than two users at a time which could be a huge help for offices.-- Joolin, Oct 29 2009 Hmm.. good points - perhaps the solution would be building new software specifically around the collaborative aspect. Companies could release alternate, collaborative versions of their software just for that purpose.-- Joolin, Oct 30 2009 random, halfbakery