My main ruler is my 6-1/2 x 24".
For specific projects, though, I generally make my own rulers - I create the grid I want on the computer, and then print it off on a transparency. I then tape it to a piece of plexiglass (I bought a bunch of different pieces on e-bay for a couple of dollars.)
That way I always have exactly the ruler I want for the project... and I only need to save the transparencies, so they take up less space.
Some of my rulers:
http://www.3dquilt.me.uk/homepagefiles/rulers.htm