These links will take you directly to the portions of databases that contain materials relevant to Mathematics. However, this is by no means an exhaustive list. Feel free to explore the General Collections to find additional resources.
Open source textbooks that are written by professional content developers who are experts in their fields and undergo a rigorous peer review process. All textbooks meet standard scope and sequence requirements.
The Open Textbook Library provides a growing catalog of free, peer-reviewed, and openly-licensed textbooks. The Open Textbook Library is supported by the Center for Open Education and the Open Textbook Network.
Bookboon.com contains free eBooks and textbooks that can be downloaded as a pdf, no registration needed. These books cannot be altered, but can be freely printed, shared, or embedded in non-commercial ways. They are intended only for individuals, colleges, and schools.
AMSER (the Applied Math and Science Education Repository) is a portal of educational resources and services built specifically for use by those in Community and Technical Colleges but free for anyone to use. AMSER is funded by the National Science Foundation (NSF) as part of the National Science Digital Library, and is being created by a team of project partners led by Internet Scout.
Founded in 2002 by Nobel Laureate Carl Wieman, the PhET Interactive Simulations project at the University of Colorado Boulder creates free interactive math and science simulations. PhET sims are based on extensive education research and engage students through an intuitive, game-like environment where students learn through exploration and discovery.