I am surprised that no one has mentioned Robert Doisneau. A lot of my favorite images of his, some of which are among my favorite images bar none, were squares.
Diane Arbus, again, is fantastic to consider, not just for the content, but the composition.
ChrisCrawford- I was under the impression that Diane Arbus shot with a Rolleiflex. Wasn't the C330 introduced after or around the time of her death?
As to composing when using a square, if you intend to keep your prints square, you have loads of flexibility. With a square, it is more acceptable to center your subject. The 'rule of thirds' is definitely more flexible. I do think it is more important to not chop your corners at a 45 degree angle than it is with rectangles though. Vignetting, in my opinion, works much better with squares. Basically, it is more important for the image to look right than look correct, if you follow my meaning. Also, keep in mind that if you get an image that would work better as a rectangle, don't feel any shame in cropping it. This is thoroughly unscientific, but I would certainly bet 20 bucks that the majority of 6x6 images get cropped to an effective 6x4.5 on the paper.