The official Marvel site has a video section that is featuring a lot of Marvel cartoons from the last two decades.  Highlights include Spider-Man (1967 series), The Avengers: United They Stand, X-Men: The Animated Series and lots more.  Other than an annoying logo in the bottom right corner they’re free and nostalgic; I can’t believe how bad some of the animation is.  Be sure to check out the live action Japanese Spider-Man.