I'm a big fan of game mods. Sometimes the release version of a game isn't quite up to par with what it could have been. (Heroes of Might and Magic V I'm looking at you ::sigh::)
Elder Scrolls IV: Oblivion was an awesome game, and one that I seem to keep "coming back to" between other games. It was really the first RPG out for the 360 and the size of the world is pretty amazing. One of the major annoyances was that the developers instituted a "level scaling" monsters/enemies. This means that as your character leveled up and became stronger, the enemies leveled up with him. While this might sound good in theory it got kind of ridiculous fighting a "Super Rat" or Skeletor when you were a high level. The point of leveling up is to strengthen your character to some serious kick-assery later in the game. Level scaling basically made that obsolete.
This is one way that Oblivion won out as a PC game over the XBox 360 version. The PC version was moddable by fans who have made a slew of good additions. Eliminating level scaling, or limiting enemies to scale only to a maximum level improved the game greatly.
However not all mods are as practical. Take for instance this "Disco Spell." Certainly good for a laugh :)