Free Booklets and Posters for Secondary Science Teachers

I have used this site for great free publications for several years.  My favorites are Inside the Cell, The New Genetics, and The Structures of Life.  The pictures are great, and the examples are current.  I order a class set of these once a year, and give them to my students as a resource.  If they turn them back to me at the end of the semester, then I give them a little extra credit.  The booklets last longer that way, but the students can keep them if they want.  Another good point is that the text and images are not copyrighted, so I can search their image gallery for amazing pictures like the one below, and use them in note sets or powerpoints. 
 
http://www.nigms.nih.gov/Education/