Windows/Mac: There are nearly as many screen-sharing applications as there are people on Earth. However, sometimes you need something simple so you can fix your aunt's computer remotely. Deskhop simplifies this process by tapping into your Facebook connections.
While power users will probably have more versatile options, Deskhop is screen-sharing for the masses. Just launch the app, type in a Facebook friend's name, and you can either offer to share your screen or request to see theirs. Both parties will be directed to download a self-destructing client that will be deleted once you're done sharing. There's nothing for your relatives to clean up or figure out.
Of course, Google's Hangouts also has screen-sharing functionality built-in that may be more robust, but Google+ still isn't quite as universal as Facebook.