Here's some interesting links:
Noorderlicht is a Dutch arts organization that runs a photography festival, an online gallery, and published books. There's some really interesting work on their website.
Light Research is a website of online photo galleries, reviews of photo books, and lots of interesting articles on a variety of photography-related subjects.
Ubuweb is one of the best websites I've seen. It's a huge repository of avant-garde art objects, sound recordings, and other interesting things.
Where do I start? The Internet Archive's goal is 'universal access to human knowledge'. Here you'll find tens of thousands thousands of music concerts, lectures, texts, etc. for free and legal download. The amount and scope of their materials is staggering.
Don't miss my favorite part, the amazing Prelinger Archives of ephemeral film.