Skype for Windows Phone is now officially available in Marketplace. Download it now here.
It supports free voice and video calls to other Skype users (or cheaper calls to landlines and other cell phones with Skype Credit) along with other cool features like portrait mode video calling, support for joining audio conference calls, call-in-progress notifications and block and unblock contacts.
See more information at Big Blog or at Skype Garage.
See a nice video walk-through below: