List of Friday Night Funkin' Engines:
The base game. Like vanilla ice cream, some people find it bland while, for others, it's their personal favorite. Only version available on Newgrounds (re-uploads of FNF or even mods aren't allowed). The Newgrounds web-only edition (no source code available) includes spritemap animations for cutscenes (the Tricky Mod uses pre-rendered videos), custom keybinds, and note splashes.
One of the original modded engines, developed for the Full Ass Tricky Mod, and later separately maintained. Focused on high-level play, featuring a reworked input system, improved player stats like a results screen hitgraph, and custom keybinds. The most common engine to use in mods (even if it isn't the simplest to mod).