if you live in the 11th / 12th, definitely go for marché d'aligre.
for another ... shall we say... experience, marché barbès is definitely worth checking out. but you've got to be prepared - prepared to get trampled on by old ladies with their shopping carts, prepared to get hit on by the friendly vendors, prepared to haggle, prepared to leave with a lot more than you intended.... (reminded me of the markets in greece)
i went with my big travel backpack - the one i took to thailand with me for one month... and it wasnt enough to carry back all my exciting groceries. black tomatoes. yellow tomatoes. cherry tomatoes (3 boxes for 1 euro). spicy olives. garlic-y olives. FRESH ALMONDS (that i need a hammer to crack open). "figues berbères" (those spiky-looking fruit in the bowl). nectarines. 7 avocadoes for 1 euro. (ok, only 3/7 were good, but the rest of the produce was delicious).
and for an hour or so, you find yourself lost in a different world and forget you're in paris. kind of like riding a bike on a sunday afternoon in belleville....
anyway, the marché is open on wendesdays and saturdays. 7h30 - 15h. it's on blvd la chapelle between metros barbès rochechouart and la chapelle.
thanks for the heads-up, célia. (though i did go here once many years ago with warren).