One True Media is a site I just learned about from Buffy Hamilton, who is one of the true pioneers in educational technology. (She has a wonderful blog which you can visit by clicking here.) This is one of the better video mash-up sites in the sense that it is very easy to use and offers some very nice tools/effects for the user.
I would have to compare OTM as a cross between iMovie and Pulp Motion. It is similar in iMovie on how you have to edit a video clip, add your music and then select your transitions and effects. Although, the finished product looks like something Pulp Motion would create which gives it that very smooth/polished look.
Some other nice features are the ability to upload a variety of media such as: mpg, avi, quicktime, wmv, and jpg. Also, a user can purchase a premium account which unlocks a lot of effects/transitions, etc. The free account lets users store up 100 MB worth of data each month.
However, I'm not sure how viable a resource this would be for students. The reason for this is that there is no filtering going on. This becomes an issue when dealing w/ CIPA/COPPA compliance. Although, a user is responsible for what they post and One True Media will banish/remove any inappropriate content. Grey Areas like this need to be handled delicately and w/ the student's best interest/safety at heart.
I recommend giving One True Media a look by clicking here.
Below is my mash-up example...