I love my moleskine quadrille notebook, but it's not quite nonlinear enough for me, or for Dave Gray.
Dave came up with the idea of hacking up a moleskine memo pad with a flap, so it can hold index cards and a pen. I've further modified it with the stupid-simple expedient of clipping blank cards to the outside, and using manila tabs on the inside to sort out my GTD planner.
And there you go.
This photo's blurry, but you get a good idea of the profile of the moleskine planner.