The LaTeX package tikz-cd is helpful for drawing diagrams.
Category Theory (second edition) by Awodey. Errata available. Main book.
Categories for the Working Mathematician by Mac Lane. Often mentioned (it even has an own Wikipedia page), but probably not at an appropriate level for this group (most are not "working mathematicians").
Basic Category Theory for Computer Scientists by Pierce.
Conceptual mathematics by Lawvere and Schanuel. "Conceptual Mathematics is a good starting place; lots of pictures. A fairly strong emphasis on sets too, rather than the topos/group examples that abound in Awodey." -- Wiegley
Categories and Haskell by Jan-Willem Buurlage. "An introduction to the mathematics behind modern functional programming"
Smith is concerned about Awodey's book being too advanced for many readers, and instead suggests Sets for Mathematics or Topoi: The Categorial Analysis of Logic (see the blog comment section). See also his page with a long list of resources, where he links his "relatively accessible" introduction (and reading guidelines "for philosophers").
Other courses:
Additional resources:
Fun links: