PhET provides free interactive simulations for physics, chemistry, biology and mathematics that visualize scientific concepts.
Open Textbook Library is a repository of freely available textbooks across multiple disciplines that can be downloaded and adapted.
Online physics and chemistry instruction with interactive tools for students.
Michael Nielsen's personal notes and essays on quantum computing, quantum mechanics, and physics.
Collaborative wiki covering mathematics and physics topics with formal definitions and relationships.
Site explaining vector calculus and mathematical concepts with visual intuition.
Hypertextbook covering mechanics, waves, optics, electricity, and modern physics with problems.
Introductory college physics text and problems.
Russian physics encyclopedia providing definitions, formulas, and reference material.