Notebook with removable pages? Similar to Circa / Rollabind
Some DIY examples I have seen that allow this are the classic hipster PDA, which uses a binder clip, and other designs which use opening rings.
I'm not a massive fan of either - the binder clip is a bit too
I really like the Rollabind ( www.rollabind.com/ ) and Circa (www.levenger.com/PAGETEMPLATES/NAVIGATION/Products.asp) systems. They provide a set of disks, and specially punched paper, that allows you to snap in and out pages at any time. A major shortcoming of this system is that their proprietary paper needs to be used - the system relies on the right shaped holes. The do sell a punch to allow you to use your own paper, but its very expensive.
Are there any other systems out there?
I might try the rollabind/circa system, and see how feasible it is to cut the paper yourself by hand.