The Hoover Institution apparently has one of the world's largest collections of posters. Click here to see some of that collection.
I myself have been a fan of pre-WWI aviation posters for many years, particularly those advertising French airshows. They are of course beautiful pieces of art, but perhaps more importantly they communicate the fascination (and optimism) associated with pre-war aviation in France. In that way they are also reminders of what might have become of Europe without a "Great War."
Addendum: Apparently the webpage I have linked doesn't work for everyone. I don't know what the exact problem is.