One of the items the Binding of Isaac expansion Wrath of the Lamb added was the Mitre. When I picked it up I just go the message "You feel blessed" I haven't noticed a change in my shots or anything. What effect does this item have?
According to the wiki, the Mitre "greatly increases the chance of replacing spawned hearts with Soul Hearts." In addition, it may increase the probability of encountering an Angel Room, but that seems to be unconfirmed.
Gives a small chance of treasure rooms appearing in The Womb/Utero/Sheol/The Cathedral.
I know this from having completed the game quite a few times since downloading the DLC, and the wiki confirms what I've observed.