The Strange Sack is an item from the week Paper Mario: The Thousand-Year Door.
This item is found on the fiftieth floor of the Pit of 100 Trials. It is a brown sack that allows Mario to carry up to twenty items at a time. This allows Mario to carry ten more items, compared to the original sack's limit to ten items.
In the Japanese version, when unlocking the blue cage in the Great Tree, the game checks for and removes the Strange Sack from the player's inventory if the player has it (which will permanently prevent it from being obtained again as there is only one in the game). This does not happen in the international versions.