The sites listed below are a great place to start searching for video clips. The subjects available will vary from site to site so be sure to check more than one if you can't find what you are looking for.
IMPORTANT: Be sure to follow the instructions on how to link to and/or download these videos. You must also check to see if the owner of each video has placed any additional restrictions on its use.
Finally, try to use a popular format such as .avi, MPEG-4, .mov, whenever possible.