Alexandre Stefanov has long maintained a list of online math texts and other materials at Geocities; this material has now been moved to 110MB Hosting. Because these resources may be of interest to our readers, we present here a modified version of Stefanov's list as of November 18, 2009.

Dynamics

• Mechanism of the Heavens (1831), by Mary Somerville. This second edition was prepared by Russell McNeil
• Dynamics in One Complex Variable: Introductory Lectures, John Milnor
• Geometrical theory of dynamical systems, by Nils Berglund
• Lectures on Mechanics, Dynamics, and Symmetry, by Jerrold E. Marsden
• Introduction to Statics and Dynamics, by Andy Ruina and Rudra Pratap

Entropy

• Entropy on the World Wide Web, has links to expository articles, journals, software, and conferences pertaining to entropy.
• Entropy of Compact Group Automorphisms, by Thomas Ward
• Valuations and Hyperbolicity in Dynamics, by Thomas Ward
• Effective Hamiltonians and averaging for Hamiltonian dynamics, by Lawrence Craig Evans
• Entropy and partial differential equations, by Lawrence Craig Evans
• CLASSICAL AND QUANTUM CHAOS, by Predrag Cvitanovic
• Dynamical systems, by Shlomo Sternberg

Quantum Mechanics

• Mathematical Methods in Quantum Mechanics, by Gerald Teschl
• Quantum Mechanics, by Klaus Schulten
• Advanced Quantum Mechanics, by Jouko Mickelsson
• Foundations of Quantum Mechanics & Quantum Information, by M. B. Plenio
• Quantum Geometry and more, by Micho Durdevich
• Quantum Probability Theory, by Miklos Redei, Stephen J. Summers

Quantum Field Theory

• Comprehensive textbook on quantum (and classical) field theory, by Warren Siegel
• Quantum Field Theory, by Ben Simons
• Mathematical ideas and notions of quantum field theory, by Pavel Etingof
• Introduction to Algebraic and Constructive Quantum Field Theory, by John C. Baez, Irving E. Segal and Zhengfang Zhou (scanned pdf. extermely big 200 megabytes)
• Lecture Notes from the Quantum Field Theory program at IAS, 1996-1997
• Graduate textbooks: Field Theory, Relativity, Superspace, Strings, by Warren Siegel
• Introduction to String Theory, by Igor V. Dolgachev
• What is String Theory, by Joseph Polchinski
• Operator Algebras and Quantum Statistical Mechanics, Volumes I & II, by Ola Bratteli
• D-Brane Primer, by Clifford V. Johnson
• Introduction to Superstring Theory, by Elias Kiritsis
• Algebraic Quantum Field Theory, by Hans Halvorson and Michael Mueger

Condensed Matter

• Worked examples in the Geometry of Crystals, by HKDH Bhadeshia
• Concepts of Theoretical Solid State Physics, by Alexander Altland
• Many-Body Physics, by Piers Coleman
• Theory of Superconductivity, by Helmut Eschrig
• High-Temperature Superconductivity, by Peter Hirschfeld
• Solid State Physics, Notes, Problem Sets, & Tests by Peter Hirschfeld
• Complex Surfaces in Materials Science, with few books
• Photonic Crystals: Molding the Flow of Light, by John D. Joannopoulos, Steven G. Johnson, Joshua N. Winn, and Robert D. Meade

Special and General Relativity

• General Relativity, Nonlinear PDEs, etc., Sergiu Klainerman
• Special Relativity and Geometry, by C. E. Harle, R. Bianconi
• Relativity on the World Wide Web. Tutorials, lecture notes, and computer simulations.
• Differential Geometry (and Relativity), notes by Bob Gardner
• Lecture Notes on General Relativity, by Sean M. Carroll
• General Relativity and Quantum Cosmology, by Sean M. Carroll
• Public lectures by Stephen Hawking

Physics

• Essential Physics 1, by Frank Firk
• Introduction to Groups, Invariants & Particles, by Frank W. K. Firk
• Classical Electrodynamics, by Bo Thidé
• Classical electrodynamics and theory of relativity, by Ruslan Sharipov
• Electromagnetic Theory and Computation: A Topological Approach, by Paul W. Gross and P. Robert Kotiuga
• MOTION MOUNTAIN TEXTBOOK Hiking beyond space and time along the concepts of modern physics, by Christoph Schiller
• Fundamentals of Thermodynamics, by J. Karl Johnson
• Non-Equilibrium Statistical Mechanics, by Klaus Schulten
• Course in Electrodynamics, by Franz Wegner
• Light and Matter, by Ben Crowell
• Order-of-magnitude-physics, a book by Sanjoy Mahajan
• A Brief Introduction to Physics for Mathematicians, Igor V. Dolgachev
• Physical Applications of Geometric Algebra, by Anthony Lasenby and Chris Doran
• Group Theory for Physicists, by Predrag Cvitanovic
• An Elementary Introduction to Groups and Representations, by Brian C. Hall
• Geometric Models for Noncommutative Algebras, by A. Cannas da Silva and A. Weinstein
• Differential Forms in Electromagnetic Theory, by K. Warnick, R. Selfridge and D. Arnold
• Lectures on integration of several variables, by Michael Adler
• Methods of Mathematical Physics, by Mark Trodden
• Concepts in Theoretical Physics, by Ben Simons
• Further Mathematical Methods, by Niels Walet
• Computational Physics, by Angus MacKinnon
• Zeno meets modern science, by Zurab K. Silagadze
• Feynman's derivation of Maxwell equations and extra dimensions, by Zurab K. Silagadze
• TeV scale gravity, mirror universe, and ... dinosaurs, by Zurab K. Silagadze
• Quick introduction to tensor analysis, by Ruslan Sharipov
• Traveling Wave Solutions of Parabolic Systems, by Aizik I. Volpert, Vitaly A. Volpert, and Vladimir A. Volpert, AMS 1994, 455 pp.