If offline GTA: San Andreas is already fun, then imagine playing against your friends or other players around the world.

Advanced map editor for Doom game.
Texture visualization and creation utility for GTA San Andreas.
Modification of the Tibia client that makes the game based on the Dragon Ball anime.
Spine-chilling challenges and the comeback of Huggy Wuggy in Chapter 2 of Poppy Playtime.
Complete conversion and balancing mod for Torchlight 2.