I'm sure most people are familiar with the Internet Archive but i'd like to highlight a couple of areas.The Live Music Archive includes thousands of recordings available for download, it's probably most well known for the Greatful Dead live shows but also includes good live music from Mogwai, Godspeed You! Black Emperor, Bardo Pond, Acid Mothers Temple and Explosions In The Sky.
The Prelinger Archives is a database that includes 48,000 "ephemeral" films (advertising, educational, industrial, and amateur) available to download and are considered public domain.