Les Petits Riens is a company of social economy. They collect goods donated by people and distribute them to others in need, or sell them to use the money for social projects. I casually came across their shop last Saturday in Chatelain, saw the movement of people coming and going and decided to go in to take a look (this is their central shop, but they have many others).
This is a three stores high big shop that sells second hand goods of everything you can imagine: furniture, kitchenware, sports accessories, books, cds, dvds, hi-fi equipment, toys, clothes, you name it. I thought it was particularly interesting for people moving to Brussels, because you can find lots of furniture and things for the house for really low prices. And I believe I saw a poster saying that for large items they can carry them to your house!
Next to it they also have a shop called Bike Paradise, where they sell second hand bikes and have a poster saying "buy a bike when you arrive and we'll buy it back from you when you leave" and then the reference to ask inside for the conditions and info on this. It may be useful for those considering buying a bike here just for a short period of time!