Factoris  
   Online calculator that factorizes large numbers, specified by formula.
   http://wims.unice.fr/~wims/wims.cgi?lang=en&module=tool/algebra/factor.en&cmd=new&

The Factor Zone  
   Aimed at grade school students and teachers, includes course guidelines, worksheets, and factor tables up to 600.
   http://factorzone.tripod.com/

Robinson Primes  
   An analysis of problems relating to the numbers k.2^n+-1, primes, and factor patterns, including the Sierpinski problem.
   http://www.glasgowg43.freeserve.co.uk/robintro.htm

Sierpinski Problem  
   Sierpinski proved there exist infinitely many odd integers k such that k*2^n+1 is composite for every n. Ray Ballinger coordinates a search to prove or disprove whether k=78557 is the smallest solution.
   http://www.prothsearch.net/sierp.html

N!+-1 Factoring Status  
   Factoring efforts that have been made so far on numbers of the form n!+-1 using ECM factoring.
   http://www.uow.edu.au/~ajw01/ecm/curves.html

Sam Wagstaff  
   Article about SNFS factorisation of 3^349-1.
   http://www.purdue.edu/UNS/html4ever/9705.Wagstaff.number.html

I Love Binary, Primes, and Factors  
   Divisibility, primes and binary numbers.
   http://home.earthlink.net:80/~usondermann/binary.html

Factorization Using the Elliptic Curve Method  
   Java applet that can be used to find 20- or 30-digit factors of numbers or numerical expressions up to 1000 digits long. It also computes the number and sum of divisors, the Euler's totient and moebius functions, and the decomposition of the number in a s
   http://www.alpertron.com.ar/ECM.HTM

Fermat Search  
   Leonid Durman's Fermat number factoring site and program.
   http://www.fermatsearch.org/

The Anti-Divisor  
   A definition and description of the Anti-Divisor, and some related results.
   http://www.users.globalnet.co.uk/~perry/maths

FactorWorld  
   Dedicated to algorithms and computational results on integer factorization. Includes links to papers, downloadable software, and online resources.
   http://www.crypto-world.com/FactorWorld.html

Factorization of RSA-155  
   Announcement of factorization of a 512-bit RSA key using the General Number Field Sieve (GNFS).
   http://www.crypto-world.com/announcements/RSA155.txt

The XYYXF Project  
   A collaborative project to produce the factorizations of x^y + y^x for 1<y<x<101.
   http://xyyxf.at.tut.by/

Number Field Sieve  
   Triade systems links to papers on the number field sieve.
   http://triade.studentenweb.org/Nfs/nfs.html

Paul Herman's Factoring Page  
   A thorough summary of many major factoring methods. Includes some source code on many pages, gentle introductions to the more complex methods and further links.
   http://www.frenchfries.net/paul/factoring/

Factoring Papers  
   Links to papers on the theory and practice of factoring.
   http://www.crypto-world.com/FactorPapers.html

Factorizations of Near-repdigit Numbers  
   Factorizations of numbers composed of all the same digit except first and/or last.
   http://homepage2.nifty.com/m_kamada/math/factorizations.htm

Computing Aurifeuillian factors  
   Algorithm that finds non-trivial factors of certain numbers of the form a^b +/- 1.
   http://web.comlab.ox.ac.uk/oucl/work/richard.brent/pub/pub127.html

Factorization of F10  
   F10 = 2^(2^10) + 1 is the 10-th Fermat number. Richard Brent describes his discovery of the two largest factors.
   http://web.comlab.ox.ac.uk/oucl/work/richard.brent/F10.html

Integer Factorization  
   Paul Leyland's list of his own and other factorisations.
   http://www.leyland.vispa.com/numth/factorization/main.htm

Factoring Fermat Numbers  
   Cash prizes for new factors of Fermat numbers Fn, for n = 12 through 22.
   http://www.perfsci.com/freegoods.htm#fermat

RSA Laboratories Factoring Challenge  
   Numbers representative of those used in the RSA cryptosystem are offered for factor attempts with prizes. A Partition List challenge is also provided in order to encourage work on factoring in general.
   http://www.rsasecurity.com/rsalabs/node.asp?id=2092

Discovery of a Lost Factoring Machine  
   Built by a French amateur, E.-O. Carissan, around 1919. Shallit, Williams and Morain include photographs and references to their paper.
   http://www.cs.uwaterloo.ca/~shallit/Papers/carissan.html

Known Amicable Pairs  
   A listing of all the known pairs of numbers, each of which is the sum of the aliquot divisors of the other. Complete for smaller numbers, and extending beyond 200 digits.
   http://amicable.homepage.dk/knwnc2.htm