NASA eClips is another great site by NASA for educators. The site contains short video clips that can be used to engage students and excite them about different topics, explain a topic, or generate a discussion.
The videos are sorted by grade level (K-5, 6-8, 9-12) and have educator resources available. There is also a Teacher-to-Teacher blog and a teacher toolbox to help teachers use the video clips in class.
The videos are well done and relevant to many different subject areas. Give them a try.