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

- 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.

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