Cortex command is an Indie particle game that has a pretty neat built in editor that allows users to make custom content for the game through editing the base weapons given to you by the game. You only need basic knowledge on character/weapon sprites to fully make use of the editor itself. All the animations/gibs/effects are categorized to make it easier for the user to get what they want in game with minimum effort. Some mods have outright changed how you play the game.
Another example is a game as old as blockland. Halo: CE, or Halo Combat evolved has a fairly huge mod community, even for this day. To get anything in game requires a community made tool that does all the work for you. HEK, Halo editing kit. It's a bit buggy on anything that isn't windows XP, but it still works to this day. Need a model rigged? HEK gets the parameters for you and compiles a suitable bone system that you can tinker with to your liking. Need a custom particle effect? Upload it to HEK and replace the desired particle for something else.
I usually don't like comparing blockland to other games given that blockland is running off a 2004 version of Torque, but the point still stands.
