Annoying precision has many really nice posts on basic category theory. Including, I think, the Yoneda lemma, epis and monos, and the center of a group.
http://www.ncatlab.org/nlab/show/closed+monoidal+category related to cartesian closed category
http://mathoverflow.net/questions/57099/why-do-filtered-colimits-commute-with-finite-limits
http://mathoverflow.net/questions/78471/intuition-for-coends
http://mathoverflow.net/questions/83437/the-main-theorems-of-category-theory-and-their-applications
http://mathoverflow.net/questions/32071/is-the-dual-notion-of-a-presheaf-useful
nLab page on Category theory concepts