ZADR is the fictional pairing between Zim and Dib, since they are not a couple in the canon series. And probably one of the most problematic fanon couples since it inspired a lot of hate clubs, mainly called Anti-ZADR. Even Jhonen is disgusted at the idea of two males that are supposed to be enemies becoming lovers.
The style in which it is represented varies widely. Whereas, some of the artists while making fan art often try to reproduce a style close to the original artist (Jhonen Vasquez). Others attempt a more personal style which can be influenced by the American/European comic or by an animesque style (like the one seen in the manga and anime). Although, we probably see all of this in most of the other fanon couples, the ZaDR is guilty of it's own, overused clichés.
The fanfics will vary as much as the fan arts, from WAFF (Warm And Fuzzy Feeling, overly sweet style of fic) to Angst (the German word that defines fear or anxiety), equally the ratings goes from G (General audience) to MA (Mature audience). Mature-rated fanfictions of ZADR is uncommonly found around the internet.
There are several clubs dedicated to this couple: