Mathematica

Advanced Mathematics for Applications
Mathematica notebooks created by Gary Stoudt for teaching and illustrating concepts in the course "Advanced Mathematics for Applications" at Indiana University of Pennsylvania. Most have some discovery learning built in. Free to download.
http://www.ma.iup.edu/projects/CalcDEMma/advmath.html

Analytic Cycling
Analytic formulas related to cycling are presented and explained. Formulas are evaluated interactively based on data supplied by the user. A MathScript web site.
http://www.analyticcycling.com/

Artlandia
Mathematica-based software for creating mathematical and algorithmic art, as well as mathematical recreations and active learning through art. Allows you to write simple programs that generate geometrical patterns, ornaments and designs. Easily creates fractal effects. Features built-in tools for "automatic" generation of attractive patterns.
http://www.artlandia.com/

Calculus and Mathematica
A Mathematica-based calculus course.
http://www-cm.math.uiuc.edu/

Combinatorics and Graph Theory with Mathematica
Combinatorica is a library of 230 functions turning Mathematica into a powerful tool for graph theory and combinatorics.
http://www.cs.sunysb.edu/~skiena/combinatorica/

Differential Equations and Mathematica
An online companion for differential equations contains demo notebooks, packages, and links.
http://www.math.armstrong.edu/faculty/hollis/mmade

DirectMath
A mathematical word processor and easy to use front end for Mathematica.
http://www.directmath.com/

The Economics Pack
The Pack integrates more than 50 Mathematica packages and more than 1000 functions in economics, statistics, econometrics and system enhancement. It provides a working environment that is basic but very useful.
http://www.dataweb.nl/~cool/TheEconomicsPack/index.html

FeynArts
FeynArts is a Mathematica package for the generation and visualization of Feynman diagrams and amplitudes.
http://www.feynarts.de/

Galois Field Package
Allows the use of many Mathematica functions over finite fields without any modification; e.g solving linear equations, inverses, determinants, derivations, resultants. By Ryoh Fuji-Hara, University of Tsukuba. Package and documentation are available for download.
http://infoshako.sk.tsukuba.ac.jp/jcca/GaloisField/

Graphica Book Series
Unusual color illustrations created using Mathematica.
http://www.graphica.com/

HostSRV.com
Internet hosting specializing in webMathematica services.
http://www.hostsrv.com/webmaa/app1/MSP/webm1003/HostSRVHome.msp

International Mathematica Symposium
User conference that invites contributions from all scientific and mathematical disciplines.
http://www.internationalmathematicasymposium.org/

Karl's MMA page
Benchmarks and timings.
http://www2.staff.fh-vorarlberg.ac.at/~ku/karl/mma.html

Linear Algebra Tools
A group of tutorial notebooks to assist students studying Linear Algebra topics.
http://www.math.duke.edu/education/ccp/materials/linalg/mathematicalist.html

LiveGraphics3D
Java applet for displaying and rotating three-dimensional Mathematica objects. Also has the ability to calculate stereo images, show animations, and embed hyperlinks.
http://wwwvis.informatik.uni-stuttgart.de/~kraus/LiveGraphics3D/

The Math Forum: comp.soft-sys.math.mathematica Discussion
A moderated newsgroup that focuses on the software package Mathematica.
http://mathforum.org/kb/forum.jspa?forumID=79

MathConsult Showroom
Various Mathematica-related demonstrations.
http://www.mathconsult.ch/showroom/showroom-E.html

Mathematical statistics with Mathematica
mathStatica solves the algebraic / symbolic problems that are of primary interest in mathematical statistics. It does so by building upon the symbolic computational power of Mathematica.
http://www.mathstatica.com/

Mathematica-Users
Provides many examples of usage of Mathematica. Users can browse Mathematica notebooks and download them. The site integrates wiki and webMathematica technology enabling computation within wiki pages.
http://www.mathematica-users.org

MathTensor
A Mathematica-based package for performing tensor analysis.
http://smc.vnet.net/mathtensor.html

MATHwire
An electronic newsletter written specifically for the Mathematica community.
http://www.wolfram.com/news/mathwire/

Stephan Kaufmann
This site provides information about Stephan Kaufmann's Mathematica books, notebooks and packages.
http://www.ifm.ethz.ch/~kaufmann/

Symmetries and Variational Derivatives
A collection of Mathematica v.2 routines by Peter Olver.
http://www.math.umn.edu/~olver/omath.html

Ted Ersek's Mathematica Tricks
Supplementary help browser.
http://www.verbeia.com/mathematica/tips/Tricks.html

Variable Symbols, Inc.
Company specializing in consulting and training in Mathematica. Based in Mountain View, CA, USA.
http://www.variablesymbols.com/

Verbeia
Economics and econometrics in Mathematica.
http://www.verbeia.com/mathematica/

Wavelet-based Time-Frequency Analysis
A Mathematica package to dissect signals found in a wide variety of disciplines in terms of their time-frequency content.
http://www.ffconsultancy.com/products/CWT/

Wolfram Demonstrations Project
A collection of thousands of interactive mathematical demonstrations created using Mathematica, all of which come complete with source code.
http://Demonstrations.wolfram.com

Wolfram Library Archive
Resources that document various capabilities of Mathematica, such as: articles, books, conference proceedings, courseware, user-submitted MathSource notebooks, and technical notes.
http://library.wolfram.com/

Wolfram Mathematica Documentation Center
Full information on the functions, capabilities, and unified architecture of the Mathematica system.
http://reference.wolfram.com

Wolfram Mathematica Player
Free interactive viewer for Mathematica notebooks.
http://www.wolfram.com/products/player/

xAct
Free Mathematica packages for efficient tensor computations.
http://metric.iem.csic.es/Martin-Garcia/xAct/