One of the best organic farmer's markets I've ever been to. Amazing cheese, bread, vegetable stalls, even a stall selling all sorts of mushrooms in season (bought some chanterelles and a black truffle last time). Smoked mozzarellas, huge focaccias, stunning veg - a truly superb market. 9am - 4pm Saturdays (the rest of the week it's a flea market).
Noordermarkt is in the shadow of Noorderkerk, about 10 minutes' walk from Centraal Station, on the edge of the Jordaan district.
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It's at the top end of Prinsengracht.
There is a general market on Saturdays on Lindengracht (parallel to Noordermarkt) selling everything from the mundane to the exotic and on Monday mornings the market specialises in cheap and cheerful materials.
Both are great places to mingle with the locals.