Combinatorics section of the Front for the Mathematics ArXiv.
A note on Catalan numbers by Ivan Galkin.
In the Mathematics Archive at University of Tennessee, Knoxville.
With Applications in Computer Science by Stasys Jukna.
Searchable database of structures with specifications and counts.
A hypertext by A. Betten, H. Fripertinger and A. Kerber.
Brendan McKay's collection of miscellaneous tables in graph6 or sparse6 format.
Comprehensive catalog of websites relating to Combinatorics.
Introduction to generating functions with interactive java applets by Alexander Bogomolny.
Iwan Jensen counts polyominoes (aka lattice animals), paths, and various related quantities.
An unsolved combinatorial problem by Frans Faase.