Many of the reports, proceedings and other sources published by the National Academies Press can be downloaded for free in PDF format. Search the Geology collection. Site requires that you provide name and email.
From the National Institute of General Medical Sciences, you will find basic quizzes and puzzles about cells, genetics, proteins, bacteria, noble prizes, inside cells, chemistry of health and much more.
Looking for free photographs to use in the classroom? All images produced or published by the USGS are in the public domain and can be freely used without permission*. Please acknowledge the USGS as the source.