Isaac is followed by a small bag that drops a coin upon completion of every two rooms. Usually a Penny, sometimes a Nickel, and rarely, even a Dime.
Money Equals Power - Allows you to increase damage much more quickly.
If you already have Little Steve or Sister Maggy with you when you pick this up, he will bug out and turn into Brother Bobby until you leave the room. If you already have Sack of Pennies and pick up Little C.H.A.D. the Sack of Pennies will turn into Little C.H.A.D. but still have both effect.