Lie superalgebras
- Irving Kaplansky's material on Lie superalgebras.
- http://justpasha.org/math/links/subj/lie/kaplansky/

History of mathematics
- Notes by Donald Lancon, Jr. Contains a brief history of calculus and other interesting stuff.
- http://www.obkb.com/dcljr/mathemat.html

Why study calculus?
- A brief look at some of the history of mathematics by Eric Schechter.
- http://www.math.vanderbilt.edu/~schectex/courses/whystudy.html

Graph Theory
- Hyperlinked PDF version of a book by Reinhard Diestel.
- http://www.math.uni-hamburg.de/home/diestel/books/graph.theory/

Algebra notes
- Roy Smith's course notes in linear algebra, elementary abstract algebra, basic graduate algebra, and a survey of the Riemann-Roch theorem from Riemann to Hirzebruch.
- http://www.math.uga.edu/%7eroy/

Probability guide for non-mathematicians
- Probability theory basics, applications (including to gambling), interpretations, books, free texts.
- http://probability.infarom.ro/

Complex Analysis
- "A First Course in Complex Analysis" by Matthias Beck, Gerald Marchesi, and Dennis Pixton is a set of lecture notes for a one-semester undergraduate course, relying on as few concepts from real analysis as possible. Numerous exercises are includ
- http://math.sfsu.edu/beck/complex.html

Linear Algebra and Applications
- Textbook by Thomas Shores. GIFs of pages.
- http://www.math.unl.edu/~tshores1/linalgtext.html

Finite Applied Associative Algebra (semigroups) & Digital Networks
- A compendium of papers by Nico F. Benschop on abstract algebra, number theory, computer science and physics.
- http://home.claranet.nl/users/benschop/

The FEMCI Book
- Finite Element Modeling Continuous Improvement. A web text by Ryan Simmons.
- http://femci.gsfc.nasa.gov/femcibook.html

Voting, Arbitration & Fair Division: Mathematics of Social Choice
- An introduction to `social choice theory', which uses mathematics to study the strengths/weaknesses of voting systems, arbitration schemes, and other methods of group decision making. Lots of pictures, requires only basic linear algebra. Author: Marcus Pi
- http://xaravve.trentu.ca/pivato/Teaching/voting.pdf

Linear Partial Differential Equations and Fourier Theory
- Complete textbook by Marcus Pivato on the solutions of linear partial differential equations, with emphasis on eigenfunction methods (and hence, Fourier analysis). Lots of pictures.
- http://xaravve.trentu.ca/pde.pdf

Calculus
- Textbook by Gilbert Strang in pdf.
- http://ocw.mit.edu/ans7870/textbooks/Strang/strangtext.htm

Measure Theory
- A book by D.H. Fremlin. Plain tex sources.
- http://www.essex.ac.uk/maths/staff/fremlin/mt.htm

Complex-Analytic Geometry of Complex Parallelizable Manifolds
- Monograph by Jörg Winkelmann.
- http://www.iecn.u-nancy.fr/~winkelma/mirror/unibas/cplx/papers/gca.html

Mathematical Methods of Engineering Analysis
- Book by Erhan Çinlar and Robert J. Vanderbei in PDF. Topics covered: functions on metric spaces, differential and integral equations, convex analysis, and measure and integration.
- http://www.princeton.edu/~rvdb/506book/book.pdf

Entropy of Compact Group Automorphisms
- Lecture notes based on a course at Ohio State in 1994 by Thomas Ward in DVI and PostScript.
- http://www.mth.uea.ac.uk/~h720/lecturenotes/

Abstract Algebra Notes
- A collection of short notes by Bruce Ikenaga in PostScript.
- http://www.millersville.edu/~bikenaga/absalg/absanote.html

Open Math Text Lecture Notes
- Lecture notes on various topics (English and Spanish) in Word and PDF format.
- http://openmathtext.org/downloads.html

Stochastic Calculus
- A fairly complete elementary introduction to the basics of stochastic integration with respect to continuous semimartingales by Alan Bain. All the theory usually needed for basic mathematical finance. Sixty pages in DVI, postscript, and PDF.
- http://www.chiark.greenend.org.uk/~alanb/

Hilbert Space Methods for Partial Differential Equations
- Textbook for beginning graduate students of mathematics, engineering, and the physical sciences by Ralph Showalter. Chapters in PostScript and PDF.
- http://ejde.math.txstate.edu/Monographs/01/abstr.html

Linearization via the Lie Derivative
- By Carmen Chicone and Richard Swanson. LaTeX, DVI, PostScript and PDF.
- http://ejde.math.txstate.edu/Monographs/02/abstr.html

Finite Fields
- An Introduction to Finite Fields by Bill Cherowitzo (html).
- http://www-math.cudenver.edu/~wcherowi/courses/finflds.html

Differential Geometry
- Lecture notes for an honors course at the University of Adelaide by Michael Murray in HTML with GIFs.
- http://internal.maths.adelaide.edu.au/people/mmurray/dg_hons/dg_hons.html

Math Wizard
- E-book gives some ideas on alternative approaches to teaching and learning math. Page includes table of contents. Requires a purchase.
- http://home.earthlink.net/~susankennison/mathwizard/

Mathematical Logic
- "A Problem Course in Mathematical Logic" by Stefan Bilaniuk in LaTeX, PostScript or PDF.
- http://euclid.trentu.ca/math/sb/pcml/

Natural Operations in Differential Geometry
- By Ivan Kolar, Jan Slovak and Peter W. Michor, originally published by Springer-Verlag in 1993. DVI, PostScript and PDF.
- http://www.emis.de/monographs/KSM/

Theory of Symmetry and Ornament
- Monograph by Slavik V. Jablan published by the Serbian Academy of Science and Arts in 1995. HTML
- http://www.emis.de/monographs/jablan/

Invariance Theory, the Heat Equation, and the Atiyah-Singer Index Theorem
- Monograph by Peter B. Gilkey published by Publish or Perish and CRC Press. Whole book or chapters in DVI and Postscript.
- http://www.emis.de/monographs/gilkey/

Matroid Decomposition
- Monograph by Klaus Truemper published by Academic Press in 1992. Chapters in PostScript.
- http://www.emis.de/monographs/md/

Spinors, Spectral Geometry, and Riemannian Submersions
- Monograph by Peter B. Gilkey, John V. Leahy and Jeonghyeong Park published by Seoul National University in 1998. DVI and PostScript.
- http://www.emis.de/monographs/GLP/

Integral And Differential Equations
- Class notes by Jim Herod at Georgia Tech.
- http://www.math.gatech.edu/~herod/4348/4348.html

Differential Equations
- Postscript notes on various topics in differential equations by Bruce Ikenaga.
- http://marauder.millersville.edu/~bikenaga/diffeq/deqnote.html

Analytic Differential Equations
- Lectures on Analytic Differential Equations by Sergei Yakovenko at the Weizmann Institute.
- http://www.wisdom.weizmann.ac.il/~yakov/thebook.pdf

Partial Differential Equations
- Lecture notes on Analysis and PDEs by Bruce Driver at UCSD.
- http://math.ucsd.edu/~driver/231-02-03/lecture_notes.htm

Ordinary Differential Equations
- Course notes by Klaus Schmitt at the University of Utah.
- http://www.math.utah.edu/~schmitt/math6410.html

Chaos
- Introduction To Chaos by Michael Cross. An online course at Caltech.
- http://www.cmp.caltech.edu/~mcc/Chaos_Course/Outline.html

Differential forms
- An introduction to differential forms and other notes by Donu Arapura.
- http://www.math.purdue.edu/~dvb/

Complex algebraic varieties and their cohomology
- A book of notes by Donu Arapura.
- http://www.math.purdue.edu/~dvb/book.html

Differential Geometry and Physics
- Lecture notes by Gabriel Lugo.
- http://people.uncw.edu/lugo/COURSES/DiffGeom/dg1.htm

Topology
- Lecture notes by David R. Wilkins at Trinity College, Dublin.
- http://www.maths.tcd.ie/~dwilkins/Courses/212/

Inverse Problems
- Lecture notes by Sze Tan.
- http://www.phy.auckland.ac.nz/Staff/smt/453707SC.html

Distributions, Fourier Transforms and Microlocal Analysis
- A short course by Yuri Safarov
- http://www.mth.kcl.ac.uk/~ysafarov/Lectures/

Group Characters, Symmetric Functions, and the Hecke Algebra
- A book by David M. Goldschmidt based on lecture notes of a course given at Berkeley in 1989.
- http://www.ams.org/online_bks/ulect4/