K - 6
McGraw-Hill Science Instructional Texts: Interactive Textbooks, Reading and Writing in Science, and Lab Books, grades 1 - 6:
McGraw-Hill: Science, A Closer Look Lab, Writing and Test Prep booklets, grades 1 - 6
Harcourt Health and Fitness Workbooks, grades K - 6
McGraw-Hill Science, 2002 Activity worksheets (graphic organizers, fill-in exercises, etc.) that can be downloaded for the matching K - 6 textbook. (When you visit the link, if you come to a username and password box, type in "technology" for both).
Thunderbolt Kids: Features grades 4 - 6 science and technology lessons that can be viewed online, or you can download here the books (which match the online material and include questions and activities), the teachers' guides (which contain answer keys and teacher notes, and corresponding science adventure comic books.
Mr. Q's Elementary Life Science: 36-week textbook, with separate student and parent editions.
Holt Physical Science Full Textbook
Holt Life Science Full Textbook
CPO Science: The following links are for full textbooks and accompanying "investigations" lab books: Life Science Text; Life Science Investigations; Earth Science Text; Earth Science Investigations; Physical Science Text; Physical Science Investigations. In addition, if you go to the For Educators page, you will find PowerPoints, graphic organizers, practice worksheets, lab investigation sheets, and illustrations to go with the textbooks.
Glencoe Teen Health Textbooks
Glencoe Teen Health Workbooks
Holt Lifetime Health: full textbook
Annenberg Learner's The Habitable Planet environmental science textbook.
If anyone knows of any other free textbooks that can be downloaded, please add the links to the comments.