Looking like an innocent barbershop, Sam's Barbershop was actually S.H.I.E.L.D. Outpost in the Lower East Side of New York City with a S.H.I.E.L.D. facility located in the sub-basement levels. It was used for a time to help train the S.H.I.E.L.D. Super-Agents but it was abandoned following an attack from the Corporation.
Years later when the Red Skull stole S.H.I.E.L.D.'s Helicarrier, S.H.I.E.L.D. returned to operating out of Sam's Barbershop as their main headquarters. After reclaiming the Helicarrier from the Skull, S.H.I.E.L.D. again abandoned the Barbershop entirely.
The empty Outpost was used for a time by Winter Soldier and Wolverine following Captain America's death. It was later one of many safehouses used by Nick Fury and his Secret Warriors until Ares discovered and subsequently demolished most of it.
In Other Media
Sam's Barbershop was featured in an episode of Avengers: Earth's Mightiest Heroes. Nick Fury brings a recently revived Captain America there and presents him with some equipment, including a motorcycle.