The Bottomless Box is an item in Dark Souls.
It can be purchased from Domhnall of Zena located in the Depths (until both bells are rung) for 1,000 souls. He can be found sitting between the way to the Gaping Dragon and the entrance to Blighttown. Since Patch 1.05, it can also be purchased from the Undead Male Merchant located in the Undead Burg for 1,000 souls.
As the name and description says, the Bottomless Box is a box that can hold an unlimited number of items. The items can be retrieved again from any other Bonfire. It works like Stockpile Thomas from Demon's Souls. However, unlike Stockpile Thomas, it does not separate every item and weapon type, and instead offers different tabs for consumables, upgrade materials, weapons, armor, ammunition, and rings.