Free AAAS (American Association for the Advancement of Science) Resources
Find Science Lessons and Tools for K-12 at http://sciencenetlinks.com/. Includes lessons, interactives, podcasts, hands-on science activities, student-centered materials.
Also check out Science in the Classroom at http://scienceintheclassroom.org/ for annotated research papers and accompanying teacher materials such as teacher guides, data activities, and curriculum connections.
Below: STEM Apps available at http://sciencenetlinks.com/.