Sites

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

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

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

Math Pages - Combinatorics (Popularity: )
http://www.mathpages.com/home/icombina.htm
About 40 "informal notes" on combinatorics by Kevin Brown.

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

Operational Research Programming Trivia (Popularity: )
http://www.ortrivia.com/
Computer code for research algorithms in operational research and combinatorial optimization.

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

The Cutting Sticks Problem (Popularity: )
http://home.planet.nl/~faase009/cutsticks.html
An unsolved combinatorial problem by Frans Faase.

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

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

Topics in Mathematics - Combinatorics (Popularity: )
http://archives.math.utk.edu/topics/combinatorics.html
In the Mathematics Archive at UTK.

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

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

East Side, West Side (Popularity: )
http://www.cis.upenn.edu/~wilf/lecnotes.html
Lecture notes by Herbert S. Wilf on generating combinatorial objects and Maple.

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

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

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

The Hyperbook of Combinatorics (Popularity: )
http://www.combinatorics.net/hyper/
A project to develop a hypertext on major topics in combinatorics.

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

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