Crapi

Crapi is an experimental multi-level PWAD developed for ZDoom from 2002 to 2006. It features approximately 20 levels, including many secret and deathmatch-only levels. It is notable in that the focus is on exploring a vast and extremely non-linear world (most levels have at least 3 exits). While some new graphics and monsters were added, level designs are mainly E1-themed, but greatly expanded and mixed with elements of natural caves and canyons.

Criticism
The packaging and release suffered from several problems which led to horrendous reviews. Most notably, it was initially uploaded to idgames as a botched PWAD with no first level, leading many to believe that the authors were either n00bs or pulling a fast one. Also, several players complained of the fact that it included the entirety of monsters.wad (a ZDoom community-built WAD featuring dozens of new monsters), even though only a few of the custom monsters were actually used.

Response to criticism
A fixed version was uploaded to idgames with the first level intact, but still containing several megabytes of extra graphic resources.

To this day, the authors contend that the whole project is very much underappreciated and would benefit from more people actually playing past the first level instead of poking around the WAD for unused resources.