Aliquot - A DOS tool to compute aliquot chains with small starting numbers, by Ivo Düntsch.
apfloat - High-performance arbitrary precision arithmetic package for C++ and Java.
ARIBAS - An interactive interpreter for big integer and multi-precision floating point arithmetic with a Pascal/Modula like syntax. It has several builtin functions for algorithmic number theory.
CALC - A calculator program for doing arbitrary precision integer arithmetic, written in ANSI C and Yacc, with a number of built-in functions for number theory, by Keith Matthews.
cBigNumber - Portable C++ class for integers with unlimited range, designed to be fast, functional and simple in use.
Encyclopedia of Integer Sequences Seeker - EIS-Seeker is a libguile-based scheme interpreter devoted to the study of the Encyclopedia of Integer Sequences database. It is intended to help find new relations between sequences. C/Scheme, Posix.
Factorizer - A Windows program to find all factors of any positive integer less than 2^31 - 1; decompose numbers into their prime constituents; find prime numbersand pairs of primes; and to plot Erdos-Kac and Palmen Color histograms. Down load free demo, order full version online.
giantint - Library of routines for large integer arithmetic and number theory.
GNU MP Home Page - Free library for arbitrary precision arithmetic.
GNUBC - Supports arbitrary precision integer arithmetic calculations with interactive execution: it is a superior version of the basic bc program which comes with the UNIX operating system. Keith Matthews has written several number-theoretic programs.
HiPiLib - High Performance Libraries - Home of PIOLOGIE, a library for arbitrary precision arithmetic, operating on natural, integer and rational numbers.
Java PHASER Project - An application for numerically and graphically analyzing dynamical systems.
KANT / KASH - KANT is a software package for sophisticated computations in number fields and in global function fields. With KASH you are able to use the powerful KANT V4 functions within a shell.
Kaprekar Series Generator - A java application for determining the Kaprekar Constants and Series for numbers in various radixes.
L - A C++ class library, datasets and command line program for computing zeros and values of L-functions, by Michael Rubinstein.
LiDIA - C++ Library For Computational Number Theory.
Magma Computational Algebra System Home Page - A large, well-supported software package for computationally hard problems in algebra, number theory, geometry and combinatorics.
NTL: A Library for doing Number Theory - A high-performance, portable C++ library providing data structures and algorithms for manipulating signed, arbitrary length integers, and for vectors, matrices, and polynomials over the integers and over finite fields.
SAGE - System for Arithmetic Geometry Experimentation: a framework for number theory, algebra, and geometry, initially designed for computing with elliptic curves and modular forms, by William Stein. Open source under GPL.