Wirt's Bell is a legendary item used to access Whimsyshire. It can be purchased from Squirt the Peddler in Act II for 100,000 gold. Wirt's bell is always available from the vendor; unlike the other items used to craft the Staff of Herding.
Wirt was a young salesman in Diablo I who was mutilated by the Fallen within the bowels of Tristram Cathedral. He has left his mark on all three Diablo games; in Diablo II, his corpse can be found within the ruins of Tristram, and his leg is used to open the Secret Cow Level.