Mobs & Entities / Potion IDs: http://minecraft-ids.grahamedgecombe.com/entities

Download Skins:

http://www.minecraftskins.net/
http://www.minecraftskins.com/
http://mcskinsearch.com/browse

Skin Editor:

http://www.silvergames.com/minecraft-skin-editor
http://www.minecraftskins.net/skineditor
http://www.minecraftskins.com/skin-editor/

Resource Packs & More Skins:

http://novaskin.me/
http://www.worldofminecraft.com/Downloads
http://www.minecraftdl.com/
http://www.minecrafttexturepacks.com/
http://www.curse.com/texture-packs/minecraft/downloads