They might not be Cannes winners yet, but over at the Viacom Outdoor, Underground website there is a large collection of PDF's of ads competing in the underground writing competition. Almost all entries are there, sadly mainly made available as downloadable PDF's which can seem a tad annoying at first, but they probably did that to let the art direction be. Click on the following images to reach the PDF files of the ads. See also Crack a smile, Harrod's tea story, national blood service... and many more.
Hat tip to adrianhon at Metafilter.