
Blanka is an interesting website that displays a collection of original, vintage and limited edition posters and prints, and has a facility to purchase them.
Someone forwarded me a link to the latest set of posters on Blanka's flickr photostream, and it's pretty awesome - a selection of work from some of the great designers of the International Style, including Neuburg, Hofmann, Kaptizki and a personal favourite of mine, Josef Müller-Brockmann.
Brockmann pioneered the theory and use of structured grid systems, and was founding editor of New Graphic Design in the late 1950's.
What I like about these posters is how little variation there is in typeface (in fact all of it is Akzidenz-Grotesk, an early precursor to Helvetica), and for these guys the use of whitespace was all about building angles and partitioning the page into sections. This allowed them to create order, producing a controlled visual narrative in their designs.
Heavy on precision, easy on the eye.

More:
Joseph Muller-Brockmann @ Blanka
Brockmann Interview @ Eye Magazine
Akzidenz-Grotesk @ wiki