As always, great tips from Free Technology for Teachers!
One of the suggested activities for flipped lesson is to have students submit videos to you that they locate, review, and annotate.
VideoANT allows annotations to any publicly accessible YouTube video (see how to reading his post and watching his video here ).
Vialogues is a free service that allows you to build online discussions around videos online or videos that you have saved on your computer (see how to here).