阿凡达 on Nick 

Official nickelodeon site (american). It is a bit slow on the episode front, although book 3 episode released are all somewhere on the interent, this site is only up to what they have aired on television.
密码: 阿凡达, avatar:the last airbender, official site, 隆世神通：最后的气宗, 阿凡达 the last airbender