A recent USA Today article mentioned a few niche science video sharing websites. I looked at the four sites mentioned in the article in hopes of finding something useful for my classroom.
SciVee looks the most user friendly for demonstrating science concepts in high school. The tag cloud and communities on SciVee make it easy to find videos related to a single topic.
LabAction, JoVE, and DnaTube have some professional science research videos. Some of these videos might might augment a discussion or investigation of current issues and techniques in scientific research. (I sure wish this type of sharing existed back in the early '90s when I was working on an immunofluorescence assay for Blastocystis hominis.)
Of course, Google Video is the best place to search out Google/YouTube Videos. Try searching rat dissection, for instance. Very cool.