Trackmania games have very robust track editors that let the community build and share custom tracks very easily.
The "press forward" genre (or "PFs") is one of my favorite examples of emergent level genres. Instead of challenging players to hone reflexes and maneuvers on a track, a PF beckons the player to simply hold down "forward" as a mindbogglingly complex track swirls around them. Through no skill of their own, a player ends up executing amazing stunts -- spinning 1080 degrees in the air before barely grazing a ramp in just-the-right-way to land perfectly on the track below. If the player makes any kind of choice, like letting go of the "forward" key, or (god forbid) turning left by 0.1 degrees, the consequences are often fatal.