Not so much a museum as a collection of murals dotted around the Quadraro neighborhood in the city’s southern suburbs. Works by nearly two dozen street artists adorn the quarter's walls, ranging from a creepy 'baby Hulk' to a striking display of wasps on Via del Monte del Grano. Many of the works are pretty faded, but if you're in the area, it's worth having a snoop around. Get a map from the MURo website.
A good place to start is with the concentration of murals along the walls of Via dei Lentuli.