I'm a little late on the "serious, serious pizza" thread, but I feel it's my duty to mention Stella's.
Stella's is on Mt. Auburn st. in Watertown in the "little armenia" neighborhood called "coolidge square," about a mile outside of Cambridge.
Stella's is classic, very thin crust, italian pizza. Available by the slice ($1.50) or whole pie. Very honest, good, old italian style. Compare to Regina's.
Open til 9 pm every day, I believe. A gold mine!