Apple Tree Supermarket
Apple Tree Supermarket is a grocery store on Amsterdam Avenue at 121st St, right next to Plimpton Hall. It's the supermarket of choice for residents of EC, Wien, Plimpton, and others who live east of Amsterdam Avenue. Like Morton Williams, it's open 24 hours, and therefore perfect for late-night snacks. Similar to Morton Williams they will occasionally gouge prices (especially produce), and also offer a narrower selection of goods. The deli is decent, as well as the salad bar.
It also has movies to rent - you don't need to open an account or leave a deposit as long as you show a credit or debit card. Overall, the biggest pull is its location and accessibility. Prepare to pay a premium, and don't be surprised if you can't find what you're looking for.