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.

Numerical Analysis

• Numerical Analysis Notes, by Lucio Tavernini
• An Introduction to Multigrid Methods, by Pieter Wesseling
• Numerical Recipes, by William H.Press, Saul A. Teukolsky, William T. Vetterling, Brian P. Flannery
• Templates for the Solution of Linear Systems: Building Blocks for Iterative Methods, by Richard Barrett
• Numerical Analysis at the University of Cambridge
• Lecture Notes on Optimization, by Pravin Varaiya
• Computational geometry, mesh generation, numerical methods, by Jonathan Shewchuk
• Numerical Analysis, by Steven Elliot Pav
• Convex Optimization, by Stephen Boyd and Lieven Vandenberghe

Information Theory

• Information Theory, Inference and Learning Algorithms, by David J.C. MacKay
• A Short Course in Information Theory, eight lectures by David J.C. MacKay
• Primer on Information Theory, by Thomas Schneider
• Several books by G. J. Chaitin (not only Information Theory)

Financial mathematics

• Measure theory, Probability, Stochastic calculus, General theory of processes, Financial mathematics, lectures notes by Rich Bass
• Structural Analysis of Discrete Data and Econometric Applications, editors: Ch. Manski, D. McFadden
• Essential Principles of Economics, by Roger McCain
• Game theory examples, by Roger McCain (html)

Game Theory

• Gametheory.net, with lectures on the subject
• Determinacy in infinite games, by Jindrich Zapletal

Cryptography

• Handbook of Applied Cryptography, by Alfred J. Menezes , Paul C. van Oorschot, and Scott A. Vanstone
• Foundations of Cryptography
• Lecture Notes on Cryptography
• Cryptography and Network Security
• Cryptography and Cryptanalysis Lecture notes
• Modern Cryptography
• Cryptography and Cryptanalysis, by Glenn Durfee
• Lecture Notes on Cryptography, by S. Goldwasser and M. Bellare

Miscellaneous

• Derivations of Applied Mathematics, by Thaddeus H. Black (GNU Free Documentation License)
• Linear Methods of Applied Mathematics, by Evans Harrell and James Herod.
• Linear Algebra, Infinite Dimensions, and Maple, by James Herod
• generatingfunctionology, by Herbert Wilf
• Algorithms and Complexity, by Herbert S. Wilf
• A=B, by Marko Petkovsek, Herbert Wilf, and Doron Zeilberger
• Applied Math Textbook, by Sean Mauch beta version (unfinished)
• Mathematical Tools for Physics, by James Nearing
• Applied Mathematics, by Peter J. Olver and Chehrzad Shakiban
• Surveys in Combinatorics
• Complexity Theory, by Oded Goldreich
• Papers on Combinatorial Enumeration and Asymptotics, by Andrew Odlyzko
• Hyperplane Arrangements, by Richard Stanley
• Introduction to Statistical Signal Processing, by R.M. Gray and L.D. Davisson