There are some wonderful films available online at the National Film Board of Canada's website. The Focus on Animation page is a good place to start, with links to the Film Board's history, techniques, and key filmmakers.
Here you can see fifty films, and read a little about each one. Be sure to look at the work of Caroline Leaf, Paul Driessen, Narman McLaren, and many of the others. The newly available resource is one of the best places on the web to see influential experimental animation.