Set in a rambling converted brownstone just steps from Central Park, ISC has 40 beds in simple dorm rooms (some female-only) and a cozy, if well-worn, lounge with full kitchen. Only guests aged 18 to 35 are generally accepted. The facilities are basic, but the location is outstanding, and there's no curfew (a small deposit gets you a front-door key).
Note that large suitcases can't go up to the rooms but must be kept in a secure area by the front desk (carry-ons and backpacks are fine, though).