Subfields in the AMS MSC 2000 classification 05.
Lecture notes by Herbert S. Wilf on generating combinatorial objects and Maple.
Maintained by Bill Chen.
Combinatorics section of the Front for the Mathematics ArXiv.
A note on Catalan numbers by Ivan Galkin.
Select a type together with specific parameter values and COS will return a list of all such combinatorial objects.
In the Mathematics Archive at UTK.
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.