Customizable Player Models

Create a custom avatar using the built-in editor. Creating a hat in the editor The models are stored in unused space on your Minecraft Skin, or as a Gist on GitHub.
Upload the exported image file as your skin to load the custom model.

The additional parts can have custom textures, higher resolution skins can be used (Steve skin from Soartex Fanver - Vanilla)
Parts of the normal player model can be hidden or moved around. Examples You can even make parts of your skin "glow" like a glow squid. Glow Plugin for Bukkit/Spigot servers for changing skins ingame (not required): https://github.com/tom5454/CustomPlayerModels/releases