I know pizza. I like pizza. I eat pizza. Haymarket Pizza has had the cheapest and best pizza in Boston (or almost anywhere else) for a long, long time. Slices are $1 (!), and whole pizzas are $6. The crust/cheese/sauce combination has a flavor that needs to be sampled to be understood. The taste is both familiar and unique. I rarely get any toppings on the slices, as they just seem to take away from the flavor of the pizza itself. I work in Cambridge, and have often traveled over the bridge into Boston just for a slice. It's that good. I recommend this place to friends and strangers alike, and I've never had anyone report disappointment.
Pros: Best. Pizza. Ever, and inexpensive to boot.
Cons: A hole in the wall. It can get quite crowded at peak hours.