Strange Sack

The Strange Sack is a brown sack from Paper Mario: The Thousand-Year Door. It is found on the 50th floor of the Pit of 100 Trials. The sack increases the number of items Mario can hold from 10 to 20.

Trivia
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 is rather annoying since the player can only get the Strange Sack once). This doesn't happen in the international versions.