Damn I should have stuck this in my notebook post. I bought a copy of Sizes May Vary just before Christmas and I use it all the time! It's a sketchbook/reference book/notebook built for designers. It has pages and pages of templates for everything you might need, business cards, envelopes, CD's, Billboards, Bus Stops and lots more.
I'm a bit reluctant to draw into the book itself, because it doesn't feel right. All those years of teachers giving me shit for doodling all over my school books have really stuck with me. But luckily for me they've included a disc with all the content of the book, so I just print out pages whenever I want them! It's really handy.
It comes with a paper size chart, which I have hung up on my wall above my desk, and more size guides in the back.