I'm not usually one to go around singing the praises of anime, but has anyone heard of Avatar? Now, it isn't technically anime, it's created in the US for US audiences and reflects that throughout, but that's just semantics because what we're talking about is an all-round amazing show. It's now about half-way into Season 3 (the last one) and is airing on Nickelodeon or, if like me you don't get Nickelodeon, the Internets.
I haven't liked a cartoon, or hell, a TV show in general, this much since BSG came along. The writing is really quite smart, dramatic yet still humourous, the characters are all well developed and likeable and the art direction is great as well.
If you've seen it, let's hear your thoughts. If you haven't, I urge you to give it a shot.