Diophantine Equations

Hilbert's Tenth Problem - Given a Diophantine equation with any number of unknowns and with rational integer coefficients: devise a process, which could determine by a finite number of operations whether the equation is solvable in rational integers.
http://www.ltn.lv/~podnieks/gt4.html

Pythagorean Triples Etcetera - A web text by Fred Barnes on 60-, 90-, and 120-degree integer-sided triangles.
http://www.geocities.com/fredlb37/

Developing A General 2nd Degree Diophantine Equation x^2 + p = 2^n - Methods to solve these equations.
http://oas.ucok.edu/OJAS/98/T98/THIENDO.HTM

The Erdos-Strauss Conjecture - The page establishes that the conjecture is true for all integers. Tables and software by Allan Swett.
http://math.uindy.edu/swett/esc.htm

Diophantine m-tuples - Sets with the property that the product of any two distinct elements is one less than a square. Notes and bibliography by Andrej Dujella.
http://www.math.hr/~duje/dtuples.html

Bibliography on Hilbert's Tenth Problem - Searchable, 400 items.
http://liinwww.ira.uka.de/bibliography/Math/Hilbert10.html

Pell's Equation - Record solutions.
http://www.ieeta.pt/~tos/pell.html

Diophantine Geometry in Characteristic p - A survey by José Felipe Voloch.
http://www.ma.utexas.edu/users/voloch/surveylatex/surveylatex.html

Egyptian Fractions - Lots of information about Egyptian fractions collected by David Eppstein.
http://www.ics.uci.edu/~eppstein/numth/egypt/

Pythagorean Triples in JAVA - A JavaScript applet which reads a and gives integer solutions of a^2+b^2 = c^2.
http://www.hbnweb.de/pythagoras/pythagoras.html

Solving General Pell Equations - John Robertson's treatise on how to solve Diophantine equations of the form x^2 - dy^2 = N.
http://hometown.aol.com/jpr2718/pelleqns.html

Rational and Integral Points on Higher-dimensional Varieties - Some of conjectures and open problems, compiled at AIM.
http://aimath.org/WWN/qptsurface2/

Diagonal Quartic Surfaces - Articles, computations and software in Magma and GP by Martin Bright.
http://www.boojum.org.uk/maths/quartic-surfaces/

Thue Equations - Definition of the problem and a list of special cases that have been solved, by Clemens Heuberger.
http://finanz.math.tu-graz.ac.at/~cheub/thue.html

Rational Triangles - Triangles in the Euclidean plane such that all three sides are rational. With tables of Heronian and Pythagorean triples.
http://grail.cba.csuohio.edu/~somos/rattri.html

Quadratic Diophantine Equation Solver - Dario Alpern's Java/JavaScript code that solves Diophantine equations of the form Ax^2 + Bxy + Cy^2 + Dx + Ey + F = 0 in two selectable modes: "solution only" and "step by step" (or "teach") mode. There is also a link to his description of the solving methods.
http://www.alpertron.com.ar/QUAD.HTM

Diophantus Quadraticus - On-line Pell Equation solver by Michael Zuker.
http://www.bioinfo.rpi.edu/~zukerm/cgi-bin/dq.html

Pythagorean Triplets - A Javascript calculator for pythagorean triplets.
http://www.faust.fr.bw.schule.de/mhb/pythagen.htm

Hilbert's Tenth Problem - Statement of the problem in several languages, history of the problem, bibliography and links to related WWW sites.
http://logic.pdmi.ras.ru/Hilbert10/

Linear Diophantine Equations - A web tool for solving Diophantine equations of the form ax + by = c.
http://www.thoralf.uwaterloo.ca/htdocs/linear.html