I'm a pretty big fan of the Avatar animated series.2 I thought the conclusion to the series was great but there were some remaining questions left hanging. So far, it appears there's no interest in continuing the animated saga around Aang.
There is a series of graphic novels (nèe comics) that continues the story and fills in some of the gaps. [Spoilers below]1
The Promise describes the events immediately after the conclusion of the animated series when Zuko begins his life as an administrator of a super power. It's more interesting than I make it sound. I just don't want to give it away
The Search picks up after The Promise and is the adventure of Zuko's pursuit of answers about his mother. It involves a schizophrenic lightning bender, so it's pretty much my favorite.
The Rift part 1 and part 2 (part 3 is not available yet, nor is the hardback) are fun but are more of an additional set of stories rather than completing the original arc. The artwork is a variation on the original and more cartoonish and less interesting to me.
All of these works carry on the aspects of the Last Airbender story line, but importantly they are consistent with the character development. Toph is still grumpy and Sokka is still the comic relief. Everything I enjoyed about the original series, is satisfyingly wrapped up in these volumes.