Mathematics

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

• Dictionary of Algorithms and Data Structures from NIST
• The MAA Online Calculus Text.
• Game Theory
• TheMacTutor History of Mathematics archive
• 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
• Chaos
• Guide to Scientific Computing from Barcodes Inc.
• Dr. Math can help you.
• Cool Math
• Mudd Math Fun Facts
• On Beyond Zillion by Curt Noll.
• 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.

Calculators

There are many application programs for computers, tablets, and smartphones. But there are also web sites that perform calculations.

• Desmos
• Easy Unit Converter
• Good calculators