The first circumferential highway (yes, highway; Bostonians don't drive on freeways) in North America, roughly 12 miles from downtown. Also the generic name for the state's high-tech region (even if most new high-tech companies now open up along Rte. 495 another 15 miles to the west).
It's also our equivalent of New York's Sixth Avenue - a roadway with a perfectly good name that bureaucrats are determined to change. The official reason for renaming Rte. 128 as I-95 and I-93 (depending on section) is that the interstate system cannot survive without having all of its roads connected to each other. Rte. 95 was originally going to be rammed through the heart of Boston and Cambridge; when Gov. Frank Sargent stopped the project, the 'crats found themselves with a gap between 95 north of the city and 95 south of the city. So now the Rte. 128 signs are gradually shrinking and disappearing.