My ALL TIME favorite website with videos to use is Scholastic's Study Jams
Study Jams has many Jams/videos on a variety of Science and Math topics. Many of the videos have karaoke style songs to help teach concepts. Each video also has interactive quizzes at the end of each segment. My kids love Study Jams and are always excited to hear we will be watching one!
Discovery Education is another favorite! It is a subscription site, but I know many schools who subscribe. This website has thousands of videos! The videos are broken into segments making it easier to choose the exact parts I want to use!
Wonder of the World- has great activities and experiments!
Looking forward to seeing everyone's favorites and adding some new ideas to my Science bag of tricks!