I found this little treasure collection while looking for some inspiration to create my own poster. There are 304 no-cost, downloadable posters in this busyteacher.org site. There is a prominent ad to purchase product on the pages but if you click on the poster you like, it will present you with a .jpg that can be saved or printed.
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The URL: http://busyteacher.org/teaching_ideas_and_techniques/classroom-posters/