Mathematics

There are separate pages for   Science   Numerical Methods   Engineering   Astronomy
and a separate page for Mathematical Software.

Dictionary of Algorithms and Data Structures from NIST

The Math archives has links to all sorts of resources on mathematics. This is a very good site.

The MAA Online Calculus Text.

Game Theory

Fun with Prime Numbers by Steve Litt.

TheMacTutor History of Mathematics archive

MathTools.net

The Probability Web.

National Curve Bank

The Wolfram Mathworld showcases Eric Weisstein's Treasure Troves of mathematics.

The puzzle archives might save you a lot of time.

ACM TOMS and similar info from GAMS.

NIST has lots of links and a good search engine for mathematical inquiries.

NetLib

The Math Forum.

There is a treasure house of wonderful Fortran code at Alan Miller's web site. Try this mirror for now.

HotMath - tutorial solutions to problems in leading textbooks.

The Art of Problem Solving.

Abstractmath

numerical methods (a good list of links, but getting a bit old)

Chaos

Guide to Scientific Computing from Barcodes Inc.

Dr. Math can help you.

Cool Math

Mudd Math Fun Facts

Vector Cross-Product Tutorial

On Beyond Zillion by Curt Noll.

Magic Software

Solve integrals online with Mathematica from Wolfram.

Planetary Ballet demonstrates some periodic solutions to the N-body problem. Charlie McDowell, UCSC.

Math Overflow

computer-verified mathematics

The On-Line Encyclopedia of Integer Sequences

FFT

The MIT site for FFT info.

Look for FFTPACK at netlib.

There are useful FFT routines from Joerg Arndt

Tom Irvine has posted some downloadable FFT software and nice tutorials.