Eternal Sunshine of the Spotless Mind certainly has an ambiguous ending, which you can interpret as being optimistic or somewhat bleak. Mulholland Drive is another good example, the film lends itself to many differing explanations and in contrast to Miriel I think it's a work of genius and maybe the best film of this decade so far. Perhaps Rashomon is the ultimate in ambiguity, a film which helped pioneer the idea of differing viewpoints on a situation, so you're left wondering where the truth lies.