Khaaaaaaaaaaaaan! The vaunted Khan Academy is a cornerstone of the online education movement, and its deep collection of 3800-plus educational K-12 videos is fully available in this superb Windows app. Support for some of Windows 8's cooler tricks—such as the Search Charm and the Play To function, to send video to connected televisions—add to the allure.
You can also log in with your Khan Academy account to ensure you get full credit for watching videos if you're doing the formal learning path thing. Physics, computer science, and biology have never been so accessible.