The Free Art Festival was on its 7th edition yesterday and it took place at Galeria Mônica Filgueiras, Jardins. The concept of the event goes like this: 11h gallery opens and they give you a password, you go in, take a look around the exhibition to see if there's any piece you might be interested in. 14h they'll call you by your password, you go in, literally take the piece you choose off the wall (IF it's still there) and that's it. it's yours, for free. Take my word for it, don't miss the next edition, but you will have to arrive really early (is worth it).
for more info: http://www.freeartfest.com.br/