If you haven't heard of Avatar before, it's an awesome cartoon with a strong continuing plot/storyline and a cast of really great characters. The concept is basically that there are four nations in the Asian-inspired world of the cartoon - the Fire Nation, the Water Tribes, the Earth Kingdom, and the Air Nomads. Each of them has "benders" who can control the element of their nation, using techniques that the creators modeled after real martial arts. In the whole world, there is always just one person, the Avatar, who has the ability to master all four of the bending techniques, and is reincarnated whenever he/she dies. When an Avatar needs to draw on all of those techniques at once, as well as the knowledge and power of all the past Avatars, he can go into the Avatar state and do so. The job of the Avatar is to use these skills to keep the world "in balance."
In the series, Aang, an Air Nomad and the current Avatar, is discovered in an iceberg where he's been preserved for 100 years by his Avatar state. As it turns out, during that 100 years, the lord of the Fire Nation has used his nation's power to control fire to wage war on all the other nations. He's even wiped out all of the Air Nomads...except Aang, whom no one could find. Now, Aang, who has mastered airbending but is still just a 12-year-old boy, has to master the water, earth, and firebending techniques, as well as control of the Avatar state, if he wants to stop the Fire Lord from taking over the whole world very soon. In his quest, he's helped by the two Water Tribe kids who found him (one is a waterbender), his giant flying buffalo, and, along the way, a lot of other fun folks. Of course, not everyone is fun - Prince Zuko (banished prince of the Fire Nation) and the Fire Nation armies are trying to hunt him down and capture him. But hey, that just makes for fun adventures, right?
One of the most fun (for me) parts of the series is watching the characters use their bending techniques. Each martial art-style technique is different and created to "fit" with the element that's being used - thus, water bending is flowing and graceful, air bending is kinda zippy and floaty, firebending is a bit grandiose and sharp, and earthbending is forceful and heavy. It gets even more fun as the series goes on and Aang begins to master the different techniques and use them together. And when Aang finally gets the hang of everything...well, let's just say this whole post was actually inspired by watching the very last episode of the series, which is a series of beautiful moments in drama and (fake) martial arts. Seriously, it's awesome.
So anyway, if you haven't seen Avatar? GO WATCH. :)
ETA: And 0ccam has now reminded me that there is going to be an Avatar movie in 2010! Yay!