Armor Stand

Note: This block is now a feature in Minecraft 1.8 Minecraft Wiki: Armour Stand
The Armor Stand allows the display of armor. When right-clicked, it presents the user with an interface showing the armor currently equipped on the stand and on the user, as well as the user's inventory.

Without a redstone signal you can shift + click on the piece of armor you want to equip. Apply a redstone signal to the stand and all the armor will be swapped by shift + right clicking rather than just single pieces.