There are now a huge number of videos on the Interweb that are suitable for classroom use, even assuming that your school blocks YouTube.
Here are a few I have found:
Open Video - University of North Carolina collection (4000+) - search by genre, duration or collection, and includes NASA films
5 min - Education (4312), History (1324), Languages (367), Science (2651) in Knowledge category. Arts include Dance, Design, Filmmaking, Drawing & Painting, and Software tutorials, and other general categories include Fitness, Food, Health, Spiritual, Sports and Travel. There are also many music tutorials
WatchKnow - Art (1301), Science (3712), Maths (2512), Eng Language (1931), Literature (1168), History (2449), Social Studies incl Geog (1192), Languages (851), PE & Health (415), The Arts (1302), Philosophy, Religion & Ethics (728) and others
NeoK12 - Thousands of vetted videos, games & puzzles for schools by subject, excluding languages, from many sources
My Learning Tube - Reviewed educational videos by subject (795)
TeacherTube - Videos (118,100), some indexed via Channels, plus Documents (2064), Audio (2304), Photos (3776) and Educational blogs (670)
SchoolTube - Moderated school, teacher & pupil video sharing website for pupils by subject
