Midori's ruler is brilliant. Fully expanded, this ruler measures (excuse the pun) 30 cm. But the really quirky thing happens when you're finished measuring, you can just fold it in half and put it in your pen case. When you then want to use it again, you just fold it out, whereupon the two parts are buttoned together so that you can easily draw long straight lines. One of the sides of the ruler has the zero mark right at the edge, so you can hold the edge of the ruler directly against what you want to measure. The ruler also acts as a protractor, with markers at every 15 degrees.
Also available in clear or pink.
See also the model in aluminium.
This ruler won the Good Design Award in 2013.