One of my favorite things on the Internet is the Internet Archive. It's actually one of the things that drew me to the field of Library Science. Citing the burning of the library at Alexandria and the recycling of old films as inspirations, the IA exists to preserve our cultural artifacts.
One example is music. The IA hosts an impressive collection of live recordings from a number of bands. Sometimes the bands themselves submit the recordings. Other times, fans submit recordings of shows that they made after the band has submitted a letter saying it was ok. Some bands, such as Defiance, Ohio) also host albums that they've made available under a Creative Commons license. Another example is movies. The IA hosts an equally impressive collection of films, both feature length and otherwise. Old cartoons are also in the archive, including Betty Boop.
Here are some specific collections the IA hosts:
Grateful Dead - The IA has an agreement with the people that represent the Grateful Dead. Under this agreement, the IA can host a huge collection of live recordings of the band.
Prelinger Archives - The Prelinger archives are an assortment of ephemeral films collected by Rick Prelinger. The collection includes industrial training films, educational films (including the infamous "Duck And Cover") and drive-in movie ads. The IA site includes a link to the archive's official site, which features a downloadable guide to the ephemeral films in the collection.
The Way Back Machine - One thing the IA was created to do was to archive webpages. It does this through the Way Back Machine. According to its site at the IA, the WBM is a collection of over 150 billion webpages with the various versions archived as far back as 1996 to as recently as a few month ago.
Project Gutenberg - Gutenberg is an outside project that hosts its files on the IA. The project is a collection of public domain texts in a variety of languages from a variety of time periods and places covering a variety of subjects. Named for the man who invented the printing press, the project collected these works so anyone could read them without having to pay. After all, why should you have to pay to read a book that was written 200 years ago?
Librivox - I don't think this is an actual set-apart collection. Librivox provides free audio book versions of texts found on Gutenberg. The audio is recorded by amateur volunteers, so the sound quality and voices aren't always the best, but it's an interesting alternative to professional audiobooks.