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Works on Windows, OS X, iOS and Android.
Pro Part of Windows
If you're on Windows XP or later, Remote Desktop Connection is already installed on your machine.
Con Not available in the Home edition (only as clients)
Con It's not for screen sharing, but for remote management
Microsoft Remote Desktop is developed for mostly enterprise users for remote control, not to share the screen with a friend. It lacks firewall support, and is much more complicated to setup properly than some other apps, designed for fast and easy sharing.