The More Player Models Mod adds the age old RPG element of character creation to Minecraft. It allows you to customize near every feature of your player and even allows you to play as any of the mobs in the game, including the Enderdragon!
You can adjust certain features to your default character such as a beard or wings and you can even change the scale of each of your limbs.
Obviously this mod would be better suited for a multiplayer server so each player can have their own, easily created, character. But it works just fine for single player use. You can even save your customizations so you can swap different character models without having to recreate your settings each time.