Susan Lenox has posted a collection of vintage German posters, illustration and advertising art (mostly late 19th and early 20th Century) to a Flickr set.
There are travel posters, ads for beer, bicycles, theatre and pens, and a variety of artists and styles. Many of the images are linked to larger versions, but unfortunately not much larger.
You can still enjoy the images though, and the interesting out-of-context subjects that can leave you wondering what some of them are about.
As in any collection of material like this, some are more interesting than others and you have to do a little digging for the best ones.