Books

A list of math books that are freely available online.

General

MIT OpenCourseWare - http://ocw.mit.edu/index.htm
A HUGE list of free textbooks - http://people.math.gatech.edu/~cain/textbooks/onlinebooks.html 
Euclid's Elements - Books I thru XIII -  Euclid - http://aleph0.clarku.edu/~djoyce/java/elements/toc.html 
Free Math Books - http://www.freebookcentre.net/SpecialCat/Free-Mathematics-Books-Download.html
Free e-Books - Mathematics - http://www.e-booksdirectory.com/listing.php?category=3
Khan Academy - http://www.khanacademy.org/ 
Math Forum at Drexel - Internet Mathematics Library - http://mathforum.org/library/  
Books from The Trillia Group - http://www.trillia.com/ 

Linear Algebra

A First Course in Linear Algebra - Robert Beezer - http://linear.ups.edu/download.html 
Applications of Linear Algebra - http://aix1.uottawa.ca/~jkhoury/app.htm  (wear sunglasses)

Complex Analysis

Complex Variables - Robert B. Ash and W.P. Novinger - http://www.math.uiuc.edu/~r-ash/CV.html
Complex Analysis by George Cain - http://people.math.gatech.edu/~cain/winter99/complex.html
*Visual Complex Analysis - Tristan Needham (not free) - http://usf.usfca.edu/vca//

Probability and Statistics

Concepts and Applications of Inferential Statistics - http://faculty.vassar.edu/lowry/webtext.html 
Introduction to Statistical Signal Processing - http://ee.stanford.edu/~gray/sp.html 
Introduction to Probability - Charles Grinstead and J. Laurie Snell - http://people.math.gatech.edu/~cain/textbooks/onlinebooks.html 
An Introduction to Probability and Random Processes - by Gian-Carlo Rota and Kenneth Baclawski - http://www.ellerman.org/Davids-Stuff/Maths/Rota-Baclawski-Prob-Theory-79.pdf
Think Bayes - by Allen B. Downey
Bayesian Reasoning and Machine Learning - by David Barber

Other

Computer Vision - Richard Szeliski - http://szeliski.org/Book/
Algebraic Geometry - http://www.freebookcentre.net/Mathematics/Algebraic-Geometry-Books.html 
Real Analysis - http://ramanujan.math.trinity.edu/wtrench/texts/TRENCH_REAL_ANALYSIS.PDF
Real Variables with Basic Metric Space Topology - http://www.math.uiuc.edu/~r-ash/RV.html 
Abstract Algebra: Theory and Applications - http://abstract.ups.edu/ 
Abstract Algebra: The Basic Graduate Year - http://www.math.uiuc.edu/~r-ash/Algebra.html
A Course In Algebraic Number Theory - http://www.math.uiuc.edu/~r-ash/ANT.html 
A Course In Commutative Algebra - http://www.math.uiuc.edu/~r-ash/ComAlg.html 
*Algebraic Topology (Allen Hatcher)                                 http://www.math.cornell.edu/~hatcher/
Vector Bundles and K-Theory (Allen Hatcher)                   http://www.math.cornell.edu/~hatcher/
Spectral Sequences in Algebraic Topology (Allen Hatcher) http://www.math.cornell.edu/~hatcher/
Topology of Numbers (Allen Hatcher)                                http://www.math.cornell.edu/~hatcher/
*Graph Theory - Reinhard Diestel - http://diestel-graph-theory.com/index.html 
*Graph Theory with Applications - J.A. Bondy and U.S.R. Murty - http://www.ecp6.jussieu.fr/pageperso/bondy/books/gtwa/gtwa.html 
Elementary Number Theory: Primes, Congruences, and Secrets - William Stein - http://modular.math.washington.edu/ent/