A Survey of Venn Diagrams: Various facts and figures about Venn diagrams by Frank Ruskey.
Algebraic Combinatorics via Finite Group Actions: A hypertext by A. Betten, H. Fripertinger and A. Kerber.
ArXiv Front: CO Combinatorics: Combinatorics section of the Front for the Mathematics ArXiv.
Catalogue of Lattices: A catalogue of different types with applications to packing spheres by Neil Sloane and Gabriele Nebe.
CATS: Combinatorial Algorithms Test Sets: Searchable index of problems, links and methodology maintained by the Association for Computing Machinery.
Combinatorial Catalogues: Combinatorial objects such as graphs, designs and geometries collected by Gordon Royle.
Combinatorial Data: Brendan McKay's collection of miscellaneous tables in graph6 or sparse6 format.
Combinatorics: Subfields in the AMS MSC 2000 classification 05.
Combinatorics: MathWorld article with basic definitions and links.
East Side, West Side: Lecture notes by Herbert S. Wilf on generating combinatorial objects and Maple.
Encyclopedia of Combinatorial Structures: Searchable database of structures with specifications and counts.
Enumeration of the Binary Trees: A note on Catalan numbers by Ivan Galkin.
Extremal Combinatorics: With Applications in Computer Science by Stasys Jukna.
G-Systems: Theory of chord structures to create music through algorithms by Vladimir Ladma.
Generating Functions: Introduction to generating functions with interactive java applets by Alexander Bogomolny.
Lectures in Combinatorics: An introduction to partially ordered sets, Dilworth's theorem, lattices and combinatorial geometries.
On-line Dictionary of Combinatorics: An expanding web text by Joe Fields.
Operational Research Programming Trivia: Computer code for research algorithms in operational research and combinatorial optimization.
Series Expansions: Iwan Jensen counts polyominoes (aka lattice animals), paths, and various related quantities.
The Combinatorial Object Server: Select a type together with specific parameter values and COS will return a list of all such combinatorial objects.
The Combinatorics Net: Maintained by Bill Chen.
The Cutting Sticks Problem: An unsolved combinatorial problem by Frans Faase.
The Math Forum Math Library - Combinatorics: Comprehensive catalog of websites relating to Combinatorics.
Topics in Mathematics - Combinatorics: In the Mathematics Archive at UTK.
