Critical Tits is a tradition at Burning Man where many (maybe over 2000) women ride around the playa on bicycles and the women mostly have bare or painted breasts.
It was started by a small group of women with a nod to the Critical Mass bike rides in SF, and one of the stated purposes was to celebrate the female form. It has become controversial partly due to "girls gone wild" style photographers, partly due to the party held afterwards (most men are excluded), and partly because there are guys who can't handle seeing bare (or painted) breasts.
Some people like it, some don't. I like the originally intended spirit, but the debate around it tends to be accusatorial.