www.SciencePhoto.com is great for unusual images and short videos from research institutions world wide. The images are water-marked but are still useful for teaching.
Here is a great video for stimulating curiosity about density and gases:
The video offers students an opportunity to see a demonstration that could not be done in the average secondary science classroom. As an introduction to a unit, students can brainstorm about what they think is happening too! Why doesn’t the cloud sink? What is the cloud made of?
This short video shows how the ossicles transmit sound from the eardrum to the cochlea. If a picture is worth a thousand words then a video is worth at least a million.