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.
To submit a Prezi presentation to the Knowmia library all that you need to do is copy the URL of your Prezi and paste it into the "import lesson" field in the myKnowmia tab in Knowmia. Screenshots of the process are available here.