I use youtube and teachertube often. Here are 2 videos I found useful in my classroom. The first one(from teachertube) is a grammar game for younger students, but I adapted it for older kids.
http://teachertube.com/viewVideo.php?title=Language_Arts___The_Simple_Compound_Game&video_id=141769
The second video(from youtube) was used as a student example for a video project assignment. I wanted my students to use color, black/white, silent picture, still shots, slow motion, and captions for their video. This video contained all these elements.
http://www.youtube.com/watch?feature=player_detailpage&v=PG6rHpDG-0c
As far as copyright and fair use, none of the information was new to me.
As far as the dropbox goes, it gives me and my students 1 place to store and access student videos/projects.