Sungeki no Kyojin ("Spoof on Titan") is a yonkoma—a four-panel gag manga—based on the world and characters of Attack on Titan by Hajime Isayama. It very loosely follows the events of the main series and builds up on its deep characterization, serving it with some of the most funny and tasteful comedy I've seen in a while. See all of your favorite characters of SnK, most notably the 104th Trainee Squad and the Survey Corps, play out various scenarios that you (probably) wanted to see but never had the chance to in the main series.
Although Sungeki reads and looks like a typical 4koma—i.e., quite simplistic—it
is, for the most part, pretty well done. Characters' emotions are all carefully articulated and feel true to the originals. I actually laughed out loud at some of the strips, which is not something every comedy gets out of me. It is one of the rare few instances of a well-designed setting and a good light-hearted comedy writing blending into something more than the sum of its parts. Good job, hounori!
Of course, I *very strongly advise* to catch up on the the first few arcs of SnK before taking up Sungeki to avoid numerous spoilers.