neoK12 has educational science videos and lessons for K-12 students. Resources are organized by Physical Science, Life Science, and Earth & Space. The site also has 1000s of photos organized by science topics that can be used in projects and presentations.
Applications for Science Teachers: This is another source of science video clips and lessons for K-12 science teachers. Great for teachers and also a great source for student presentations, research, wikis, blogs, etc.