libskinupload
Signal_

A skin system that allows players to upload custom skins in-game.

VoxeLibre Custom World Skins
covertiii

Add Custom Skins in VoxeLibre

Online Skins
SkyBuilder1717

Adds online skins database and site to upload your own.

BlockHead
Silver Sandstone

Lets you put a block on your head.

Held Item Models & Animations API
TomCon

A powerful API adding in an easy system to integrate full 3d models when holding items.