Screen/Window sharing software that's NOT Skype

Hello PcMasterRace users. Just a quick and simple question here. I'm looking for a screen sharing software that will allow me and my friend to share each other's window at the same time. I know of the "mirroring effect" but we aren't looking to share the whole desktop, just a single window.

At the moment we are using four skypes, open at the same time to view each others screens. Is this excessive, annoying, and difficult? Yes. That is why we are looking for an alternative.

Just in case you are wondering, "What idiots want to share a single window, and why?" The answer is that we are playing GBA roms together, and it really adds a lot of fun to the single player games when you can see each others screen side by side.

Thanks for any answers!