Windows/Mac: Screen-sharing applications are a dime a dozen these days. Screenhero differs from the rest by letting both users control the screen with their own cursors and keyboard at the same time.
Most screen-sharing applications require you to pick a "controller" and a "viewer", so only one of you can really move the mouse and type text at a time. Screenhero gives you two cursors and allows both users to control the app at the same time. Note that when we say "same time" we don't mean you can both type or both focus different windows at the same time -- we just mean you don't have to click a button to give the other permission to move.
Screenhero is remarkably good at handling who can do what at any given time: if one user is typing, it won't let the other person change focus to a new window, for example. With a bit of communication between you and your partner, it should make collaborating a lot easier. Check out the video demo above to see it in action, or hit the link below to try it out for yourself.