One of my big design inspirations is the poster work of the Works Progress Administration (WPA); a "New Deal" agency formed by the US government in the 1930s to help end depression era unemployment.
The Library of Congress has a great searchable archive of close to 1000 WPA posters (500 of them are about not getting syphilis: good to know!).
Most of the posters feature bold abstracted designs and type typical of the modernisim that is associated with the Art Deco era. Despite the often semi-professional status of the artists who made them, they have come to have their own identifiable style. The posters are a lesson in simplicity, whilst maintaining personality.
You can see their direct influence in my own posters and cover artwork for Transformers: All Hail Megatron.
You may also be interested in my article on vintage concert poster design.