I spend a lot of time doing consult work, teaching, tutoring, etc., both volunteer and paid, and over the years (and unintentionally) I started reviewing and documenting freely available sources for my students, colleagues, and persons here. I thought it would be nice to share some of these in one place, and to ask that others share any such sources they know of. These are some that I either often refer others to (and in some cases, particularly the INI seminars, use myself) or that are included in my bookmarks/favorites folders for Chrome/IE.

http://ocw.mit.edu/index.htm (a tremendously valuable source consisting mainly of taped courses held at MIT from introductory calculus to advanced chemistry)

http://theoreticalminimum.com/courses (the great physicist Leonard Susskind has taught a series of physics courses intended to provide those without a background in physics with "the theoretical minimum" instruction to really learn classical, quantum, statistical, and other physics. These lectures are available through YouTube, iTunesU, and other places, but here they are most easily accessed and best organized).

iTunesU (if you have iTunes, iTunesU contains many, many, many collections of recorded courses, seminars, colloquia, and so forth).

http://www.newton.ac.uk/webseminars/ (Isaac Newton Institute for Mathematical Sciences. Many of their seminars are taped, and range from topics like climate science to the mathematics of liquid crystals.)

http://www.codecogs.com/index.php (CodeCogs. A library of resources and code for researchers).

http://www.sagemath.org/ (SageMath. "SageMath is a free open-source mathematics software system licensed under the GPL. It builds on top of many existing open-source packages: NumPy, SciPy, matplotlib, Sympy, Maxima, GAP, FLINT, R and many more.")

http://sourceforge.net/ (SourceForge is a huge database of free apps and programs, most of which are of limited value yet hundreds upon hundreds of which are invaluable)

http://demonstrations.wolfram.com/ (for those who use the CAS program Mathematica, either to teach or to learn or as a research tool, their demonstrations project contains an enormous number of programs/apps, many of which also provide the complete source code)

http://open.umn.edu/opentextbooks/ (Open Textbook Library. The name says it all.)

http://www.freebookcentre.net/ (Another source for free ebooks. I haven't really checked this one out, so if anybody does and finds it contains pirated material, please let me know so that I can remove the link.)

http://www.freescience.info/index.php (same as above)

http://www.intechopen.com/ (a library of free academic/science volumes that I KNOW are legal; these run the gamut from nanotechnology educational theory.)

http://classicalrealanalysis.info/Free-Downloads.php (a number of free mathematics textbooks, mostly related to analysis and integration. They are superior to many a $100+ dollar textbook.)

http://online.stanford.edu/courses (like MIT's OpenCourseWare. Also the origins of Susskind's The Theoretical Minimum).

http://cow.math.temple.edu/~cow/cgi-bin/manager (Calculus on the Web. Ever since I discovered this site, every student I've taught in calculus, linear algebra, or abstract algebra has been directed to use it. It is a series of "books" that are actually just practice problems which lead the student through mathematics problem sets broken down by "book", "chapter", & "section". These books include "Precalculus", "Calculus I, II, & III", "Linear Algebra", "Number Theory", and "Abstract Algebra")

http://john-uebersax.com/stat/likert.htm (Likert Scales: Dispelling the Confusion. Actually, the links at the bottom of the page are more helpful, but my distaste for the ways in which Likert Scales are used biased me here and I used this as the base site rather than the links)

https://personality-project.org/r/book/ (An introduction to psychometric theory with applications in R, and online and downloadable e-book and more)

https://personality-project.org/r/book/ (an e-book on psychometric theory and measurement in the social & behavioral sciences)

http://www.logicmatters.net/tyl/ (Teach Yourself Logic: A Study Guide (and other Book Notes). It's actually more than an ebook and book notes. As I think everybody should take a course in logic and as this site provides not only a free text but much more, I included it. However, I suspect that most here would at best use it for teaching material.)

https://archive.org/details/texts (An enormous archive of texts all available in various forms; somewhat akin to Google Books that are free, but better.)

Cornell's arXiv. I'm not giving the link because this should be better known than is Google Scholar.

https://www.coursera.org/ (Coursera. Free courses on just about everything).

Some free ebooks (or the equivalent). I've either reviewed or used to as instructional material all of them. I've included only those that I think are of some value, but a lot of that value comes from the fact that they are free (and legal!).

http://www.arthurjaffe.com/Assets/pdf/IntroQFT.pdf (An Introduction to Quantum Field Theory by Arthur Jaffe)

http://arxiv.org/pdf/math-ph/9807030v1.pdf (Lecture notes on C*-algebras, Hilbert C*-modules, and quantum mechanics)

http://www.math.cornell.edu/~hatcher/AT/ATpage.html (Algebraic Topology)

https://www.jyu.fi/maths/en/research/stochastics/lecture-notes-for-stochastics-1/probability-1.pdf (An introduction to probability theory)

http://people.cs.clemson.edu/~steve/CW/395/CS483-part1.pdf (LECTURE NOTES ON QUANTUM COMPUTATION AND QUANTUM INFORMATION THEORY. I include this one only because of a footnote: "Mathematicians tend to despise Dirac notation, because it can prevent them from making important distinctions, but physicists love it, because they are always forgetting that such distinctions exist and the notation liberates them from having to remember.")

http://www3.nd.edu/~lnicolae/sing.pdf (Notes on the Topology of Complex Singularities)

http://webmath2.unito.it/paginepersonali/sergio.console/lee.pdf (Introduction to Smooth Manifolds)

http://people.sissa.it/~bruzzo/notes/IATG/notes.pdf (Introduction to  Algebraic Topology and Algebraic Geometry)

http://tomlr.free.fr/Math%E9matiques/Math%20Complete/Algebra/Calculus%20approach%20to%20matrix%20eigenvalue%20algorithms%20-%20Hueper.pdf (A Calculus Approach to Matrix Eigenvalue Algorithms)

http://www.math.umn.edu/~garrett/m/algebra/notes/Whole.pdf (Abstract Algebra)

http://msp.org/gtm/2000/03/gtm-2000-03p.pdf (Invitation to higher local fields)

http://samizdat.mines.edu/tensors/ShR6b.pdf (Quick Introduction to Tensor Analysis)

http://www-thphys.physics.ox.ac.uk/people/FrancescoHautmann/Ralhep/ralss10_p.pdf & http://www.stfc.ac.uk/PPD/resources/pdf/QEDQCD09.pdf (two different lecture notes/books entitled "An Introduction to QED & QCD"; I found the one while searching for the other.)

http://arxiv.org/pdf/hep-th/9912205.pdf (Fields)

I have tons of such titles, and will continue to list more but at the moment I am spent.