Mynonymouse sez, "The Library of Congress just posted a Flickr set of lovely WPA posters. There are awesome ones about keeping your teeth clean, science and one that seems to be about drunk driving but also might warn of a previously unknown deadly reactive incompatibility between gas and whiskey."
These are awesome designs, but it's a disappointment that the LoC posted them at such crummy low-resolutions. The nation's treasures deserve better than that.
Update: See the comments for lots of places where you can get higher rez ones.