Game modding.
 

So this topic covers a lot of ground. A number of games - especially on Steam let people create user-generated content and share it. Unauthorized modding is also fairly popular - ROM hacking of Pokémon, prying open game engines like Unreal 1 and 2, and so on.

Anyone have any experience with this sort of thing?

Well, I know someone who tried making a Mario hack and still hasn't showed it to me yet.

I made a terrible Super Mario World hack when I was 13. It did have a few inventive levels, but many were modifications of existing levels. It was easy to max the life counter, and the later levels and the overworld nearby were buggy. (One problem with testing the overworld is that you had to start a new game, and I had gotten sick of the early levels by then.)

I also made a few random Super Mario World custom levels. My favorite was one where the goal post was right at the start, but there was a cliff directly after it so you died and it was impossible to actually pass the level.

Not sure if this would really count, but back in the Pokemon Diamond/Pearl days I had a Action Replay for my DS and created a lot of random/stupid hacked Pokemon.

Nowadays I just mess with a trainer for GTA IV that makes the game a billion times more random (car billiards with low gravity anyone?).