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

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

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

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/

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/

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

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

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

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

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

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

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

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

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

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

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

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

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/

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

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

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

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

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