This lets you mod things server-side only (ie clients don’t need to install mods). And do so without having to compile or do much other than have a text editor. There are some limits (eg you can’t introduce new textures or models) but it’s still a lot of fun.

Mischief I’ve been up to with it:

• Make cows and horses shearable (drops leather, reverts them to baby form temporarily) by intercepting PlayerUseEntityEvent
• Disable fall damage near a particular mysterious tall tower and play a splash noise instead, by intercepting BeforeLivingEntityDamageEvent
• Add mysterious sometimes-teleporters to two sets of identical ruins (one near spawn). My friends hated this, the rules to get it to work were too arbitrary. I felt like a 90’s adventure game developer.
• Add an /easydifficulty command (sets a player’s max health higher), because some people on the server just wanted to farm, build and chill.
• A deathchest mod, where the inventory is stored in an invisible minecraft:interaction entity stretched over a sign with a head on top. The advantage of this over chests is that all the items can be restored to their original inventory slots (including cursed armor xD) as I can store arbitrary data.
• Add a /crush <player> command, which informs the target that you have a crush on them and gives them a free rose.