From Thinkgeek:

This mug lets you decide how your day’s going to go. On one side, it features the sweet Stay Puft Marshmallow Man’s smiling face, the one Ray remembers from his childhood. On the other, it features evil Stay Puft Marshmallow Man, destroyer of worlds, the one who’s come to terrorize New York City. Which side will you drink from today? Capacity: 20 oz.

[Ghostbusters Stay Puft Marshmallow Man Face Mug]