I've seen a few episodes of Avatar: The Last Airbender at a friends house. It's a great cartoon! I'm really surprised at how good it is, and that it's on Nickelodeon and meant for kids. Haha. Oh well, it's definitely a fun show to watch.
It's about Aang, the Avatar, who can master all four martial art forms: air, water, earth, and fire. He can manipulate them to do what he needs: fly, form earthen shields launch gusts of wind or water, etc. His mission: simply to save the world by reuniting the four nations (Earth, Wind, Water, Fire). For a show for kids it has a great story, in addition to the cool moves and animation (based on true martial art forms). Each season follows a story arc and each episode adds to the overall story. Very cool for a kids carton! There is a lot of great info about the show on Wikipedia.
UPDATE: According to the Wikipedia article, M. Night Shyamalan is going to make a live action movie trilogy based on this cartoon.