Yume 2kki (ゆめ2っき)

Overview
Yume 2kki (ゆめ2っき) (Yume Nikki, literally Dream Diary, but also a pun on the original game's title -- に, or "ni," means 2) is the original fangame that started it all. It is a large collaboration project from 2chan users and thus features a wide variety of worlds, effects and ideas.

Gameplay
Just like the original game, there is no overarching plot and the only goal is to explore and collect all of the effects.

A unique feature of 2kki is the inclusion of collectible wallpapers, unlocked by reaching certain areas or activating events. These can be found on Urotsuki's real world PC and serve as a large side-quest for the game. Collecting 167 (?) of the 177 unlocks one of the game's four endings.

Plot
Due to the vast and many, many worlds of Yume 2kki, plus the four endings, multiple theories have been made, and it is really up to the player on how they choose to interpret it.

Effects
Yume 2kki has a massive 34 effects, something unique about Yume 2kki is the ability to mix effects, such as Bike + Wolf.

Effect Mixes
For more information on Yume 2kki's effects and where to find them, please see Yume 2kki's Wiki.