For walks around town I love to use a 6x6 folder. I have an Ansco Titan that fits in my back pocket like an over-stuffed wallet. When I need more than guestimation focusing I use a Rolleiflex or Hassy 500cm.
Another 6x6 SLR that I've had lot's of fun with, but is sort of under the radar, is the Graflex Norita (and previous incarnations). It's a lot like a Pentax 6x7, but it's 6x6. The story I heard about it (I don't know how good my sources are) is that it was designed by the guy who designed the Nikon F. The really cool thing about it is the standard 80mm f/2 lens it usually comes with.