Fantastic Planet (1973), a feature film directed by Rene Laloux, won the Grand Prix at the 1973 Cannes Film Festival.
It is available online in 8 parts:
Senses of Cinema has an article on the film. SciFi.com reviews it here.
7/16/2009: FilmSquish has a review.
11/18/2009: Ferdy on Film has a review.