Knowmia is a free service that offers a huge library of videos and presentations for flipped lessons. The library is generated by the contributions of teachers. Users of Knowmia's free Knowmia Teach iPad app can create their own flipped lesson videos and submit them to the Knowmia library. I recently learned that you can also submit your Prezi presentations to the Knowmia library.
"Create meaningful, annotated conversation around video. Use Vialogues to supplement face-to-face instruction or as the heart of an online learning experience. A vialogue (video + dialogue) puts you at the center of an immersive video-based discussion that includes time-coded, hyperlinked comments. Teachers can post comments, polls, and surveys to scaffold video content."