Normally, to play as Shrek in League of Legends, you'd have to play a Chinese clone of the game. But now, thanks to a mod, you can play as the king of the swamp in the actual game—but only if you use Alistar.
You can see the mod in action in this video by UberDanger. PLAY TIME IS OGRE.
You'll notice that, with skins, League of Legends pretty much seems like Fanfiction: The Game. It's kind of great.
As with the Snoop Dogg mod, this skin is merely a local cosmetic change: nobody aside from you can see that you're playing as Shrek.
You can grab the Shrek skin here (click the blue "download" button). If you need help applying the skin, here's a tutorial: