Seeing a sign with the word brocante always causes my heart to quicken a bit. From what I understand, a brocante sells second-hand goods that are nicer than what you would find at a thrift store, but may not officially be classified as antiques. There are permanent brocantes throughout Paris such as the Passy-Brocante or l'object qui parle (click on the names for more info).
There are also temporary brocantes where vendors set up their wares for several days on sidewalks throughout the city, mainly from spring to autumn. I snapped a few photos to give you a flavor of what is sold at these temporary brocantes.
Sometimes you are lucky enough to just stumble upon one while you are out and about in Paris like I was last summer under Pont Alexandre III. If not, you can find a listing of upcoming brocantes (here and here). Although the sites are in French, don't worry. I have included a few tips to help you navigate them. First of all, the dates listed are not only for brocantes, but for other types of temporary sales.
For instance, the sites include dates for vide greniers, which are somewhat equivalent to an American neighborhood block sale. And they include sales for collectors of specific things such as stamps and postcards (Marché aux Timbres et Aux Cartes Postales). So, just look for the word brocante on the website.
Secondly, these sites don't always provide the exact times or locations of the sales. Generally, I try to hit a sale around 11am as many vendors close up shop at lunch time often between 1:00pm and 2pm. Some of the addresses are easier to decipher. If the sale is at a place (e.g., Place d'Aligre) it is fairly easy to pinpoint on a map. Addresses that are just street names are a bit tricker (e.g. avenue Felix Faure) as you don't know exactly where the sale is on the street. I wear comfortable shoes and expect that I may need to poke around the street a bit to find the sale. Lastly, bring cash and hopefully you will leave the brocante with some little treasure to serve as a memory of your time in Paris.