I have used Youtube to show students speeches from history. The following is Reagan's speech at the Berlin wall. I show this to students so they can make the connection between the symbolism of the wall and the height of communism.
Another tool I have used is Discovery Education. I have used this to show students primary source videos (in stations) for WWII. The following is a link to Executive Order 9066 which discusses the interment of Japanese Americans.
I already have a Dropbox account. I have not had an opportunity to use it much yet. But I can definitely see the potential. Students have uploaded videos to be shared with the school. I have also used it to share resources with other teachers.