To mark the occasion I've got a special recommendation, although it's not exactly a movie. For Earth Day I encourage you to watch Planet Earth, the eleven part documentary series commissioned by the BBC's Natural History Unit. It's an amazing show that takes you to a different part of the planet for each episode, where they look at the habitat, history and wildlife of the area. It aired back in 2006 and was narrated by David Attenborough, although when it was on the Discovery Channel in the U.S., for some reason Sigourney Weaver took over the narration duties (does it really make people want to watch a nature documentary more knowing that a celebrity's voice is talking over it?) It was actually filmed in High Def and now that the complete series is on blu-ray, looks absolutely incredible on your TV. Trust me, if you start one episode, you'll be hooked on the whole thing.
Here's a segment from the series that shows a pride of lions going after an elephant. It's crazy how they managed to obtain that footage at all: