The area I live in is full of beautiful murals. Depending on how I commute, I pass the Area 4 Story mural on my way into lab. Near Symphony Hall in Boston, there's a sepia-toned mural of artists from the Boston Symphony Orchestra. The power boxes around the city are also often decorated: my favorite is one painted like a bookshelf outside of a used bookstore in Central Square.
Here's another mural I found today while biking out to Davis Square:
If you're looking for this one yourself, it's on Park Street in Sommerville between Sommerville Ave. and Beacon St.