Adventure Backpack adds over 70 different backpacks to Minecraft. They store items and fluids, are immune to lava and fire, and keep their inventory when dropped. Also they place themselves as a block when the player dies to prevent losing the items inside.

Right-click on the ground to place the backpack down. Right-click in the air to open it and then click the "man" icon to equip it. Press 'B' to access it when wearing so that you can remove it (click icon in bottom left with a red arrow to remove it).

MORE INFO
Minecraft Forums
Project Wiki
Minecraft Mods