The Marché aux Puces de Saint-Ouen at Porte de Clignancourt is the largest flea market in the world, sprawling across several distinct sub-markets with over 2,000 vendors. About 70,000 visitors descend each weekend, which gives you a sense of the scale. Take Metro line 4 to Porte de Clignancourt and follow the crowds north.
The market is actually a collection of a dozen or so individual markets, each with its own character. Marché Vernaison is the oldest and most labyrinthine, packed with vintage curiosities and small antiques. Marché Biron is where the serious (and seriously priced) antique furniture dealers set up. Marché Dauphine is strong on books, prints, and vinyl.
Don't expect bargains on anything genuinely valuable — the dealers know exactly what they have. But for browsing, for atmosphere, and for the occasional find among the bric-a-brac, it is unbeatable. Open Saturday, Sunday, and Monday.