Life on Earth is an iBooks Textbook consisting of 41 chapters in 7 separate units and can be downloaded for free from the iBooks Store.
Lumen Learning provides a Biology text with various chapters such as The Chemical Foundation of Life and Osmotic Regulation and the Excretory System.
Lumen Learning provides a Microbiology text with various chapters such as Cell Structure of Bacteria, Archaea, and Eukaryotes and Pathogenicity.
MERLOT is a curated collection of free and open online teaching, learning, and faculty development services contributed and used by an international education community.
This is a great resource that provides textbooks on biology and life sciences.
OER Commons provides a curated collection of textbooks and resources for biological studies including zoology, botany and ecology.
Over thirty open textbooks covering Biology.
Browse a wide selection of textbooks on biological science topics.
Provides a robust search to limit by activity type, media format, and age level. (Actually a pre-defined subset of the OER Commons)
More advanced, specific science resources that are open and could be used in a classroom setting.
Funded by the NSF and organized by Library of Congress subject headings. Aimed at Community and Technical Colleges, but free for all.
A free science library and personal learning tool. Currently concentrates on genetics and cell biology.