Topics in Mathematics - Combinatorics - In the Mathematics Archive at University of Tennessee, Knoxville.

http://archives.math.utk.edu/topics/combinatorics.html

Combinatorial Catalogues - Combinatorial objects such as graphs, designs and geometries collected by Gordon Royle.

http://www.cs.uwa.edu.au:80/~gordon/data.html

The Combinatorics Net - Maintained by Bill Chen.

http://www.combinatorics.net/

ArXiv Front: CO Combinatorics - Combinatorics section of the Front for the Mathematics ArXiv.

http://front.math.ucdavis.edu/math.CO

Combinatorics - Subfields in the AMS MSC 2000 classification 05.

http://www.ams.org/msc/05-xx.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

Lectures in Combinatorics - An introduction to partially ordered sets, Dilworth's theorem, lattices and combinatorial geometries.

http://www.freewebs.com/desargues/combinatorics.htm