1. I find that Discovery Education and YouTube are the most user-friendly and would be tools I might actually use in my classroom. I tend to already use PBS, so it would be helpful to just embed the things I use into my flipcharts.
2. Here are two videos that I use regularly in my classes. The first is part of PBS' Evolution series. YouTube cuts the movie into segments, so you would have to embed all segments and click each one as the previous ones ends. That kind of sucks, but it would work in a pinch!
The 2nd one is a video I use to introduce Genetics and the work of Gregor Mendel. It is by a Biology teacher named Mrs. Harvey and she is very brave to get up and rap to her students! I love it and the kids love it too! Embedding it sure saves time hunting for the websites, etc.3. Basically, I learned that teachers are pretty well protected against copyright infringement as long as we are not selling or distributing it. In addition, if you have your stuff password protected and not available to the general public. Section 110 of US Copyright Law states that teachers are allowed to repurpose material for face-to-face instruction in a non-profit institution. Teachers have a lot of leeway that other professions do not - score one for the teachers!
4. I already have a Dropbox account and I also use Picasa for pictures. I don't believe students will use Dropbox very often because we have Google Docs. I however might use Dropbox to put flipcharts in so that I can access them from my home computer.