Combinatorics

Enumeration of Polyhedra - Enumeration of polyhedra with upto 9 faces by Steven Dutch.
http://www.uwgb.edu/dutchs/symmetry/polynum0.htm

Squared Squares - Tilings of squares and rectangles, the classical problem and variations.
http://www.squaring.net/

On-line Dictionary of Combinatorics - An expanding web text by Joe Fields.
http://www.southernct.edu/~fields/comb_dic/

Generating Functions - Introduction to generating functions with interactive java applets by Alexander Bogomolny.
http://www.cut-the-knot.org/blue/GeneratingFunctions.shtml

ArXiv Front: CO Combinatorics - Combinatorics section of the Front for the Mathematics ArXiv.
http://front.math.ucdavis.edu/math.CO

A Survey of Venn Diagrams - Various facts and figures about Venn diagrams by Frank Ruskey.
http://www.combinatorics.org/Surveys/ds5/VennEJC.html

The Math Forum Math Library - Combinatorics - Comprehensive catalog of websites relating to Combinatorics.
http://mathforum.org/library/topics/combinatorics/

Topics in Mathematics - Combinatorics - In the Mathematics Archive at University of Tennessee, Knoxville.
http://archives.math.utk.edu/topics/combinatorics.html

Lectures in Combinatorics - An introduction to partially ordered sets, Dilworth's theorem, lattices and combinatorial geometries.
http://www.freewebs.com/desargues/combinatorics.htm

G-Systems - Theory of chord structures to create music through algorithms by Vladimir Ladma.
http://www.sweb.cz/vladimir_ladma/english/music/articles/dide99.htm

Extremal Combinatorics - With Applications in Computer Science by Stasys Jukna.
http://www.thi.informatik.uni-frankfurt.de/~jukna/EC_Book/

The Cutting Sticks Problem - An unsolved combinatorial problem by Frans Faase.
http://www.iwriteiam.nl/cutsticks.html

Polya's Enumeration Theorem - An article on generating functions and Polya's theory of counting by Karl Dahlke.
http://www.mathreference.com/cmb-gf,pet.html

Series Expansions - Iwan Jensen counts polyominoes (aka lattice animals), paths, and various related quantities.
http://www.ms.unimelb.edu.au/~iwan/Series.html

The Combinatorics Net - Maintained by Bill Chen.
http://www.combinatorics.net/

Combinatorial Data - Brendan McKay's collection of miscellaneous tables in graph6 or sparse6 format.
http://cs.anu.edu.au/~bdm/data/

Combinatorial Catalogues - Combinatorial objects such as graphs, designs and geometries collected by Gordon Royle.
http://www.csse.uwa.edu.au/~gordon/data.html

Enumeration of the Binary Trees - A note on Catalan numbers by Ivan Galkin.
http://ulcar.uml.edu/~iag/CS/Catalan.html

A New Proof of the Four Colour Theorem - By Ashay Dharwadker. Includes profile, research papers for other algorithms, lecture notes, and student database.
http://www.geocities.com/dharwadker/

Combinatorics - MathWorld article with basic definitions and links.
http://mathworld.wolfram.com/Combinatorics.html

CATS: Combinatorial Algorithms Test Sets - Searchable index of problems, links and methodology maintained by the Association for Computing Machinery.
http://www.jea.acm.org/CATS/

Encyclopedia of Combinatorial Structures - Searchable database of structures with specifications and counts.
http://algo.inria.fr/encyclopedia/

Combinatorics - Subfields in the AMS MSC 2000 classification 05.
http://www.ams.org/msc/05-xx.html

The Combinatorial Object Server - Select a type together with specific parameter values and COS will return a list of all such combinatorial objects.
http://www.theory.csc.uvic.ca/~cos/

Algebraic Combinatorics via Finite Group Actions - A hypertext by A. Betten, H. Fripertinger and A. Kerber.
http://www.mathe2.uni-bayreuth.de/frib/html/book/hyl00.html

Catalogue of Lattices - A catalogue of different types with applications to packing spheres by Neil Sloane and Gabriele Nebe.
http://www.research.att.com/~njas/lattices/

The Seventeen Wallpaper Groups - Examples in traditional Japanese patterns.
http://mathmuse.sci.ibaraki.ac.jp/pattrn/PatternE.html

Counting Young Tableaux - An article on counting Young tableaux of bounded height by François Bergeron and Francis Gascon.
http://bergeron.math.uqam.ca/Bessel/bessel.html