WildCats
-- brings Category Theory to Mathematica#Mathematica
WildCats lets you manipulate and visualize categories, functors natural transformations, universal properties, etc.
This seems to be the only package for Mathematica (or any other major CAS) dedicated to Category Theory, the 70 year old, new all-encompassing mathematical language.
The latest version 0.70 has just been released.
I am actively developing it and always welcome users' comments and suggestions.