For category theory in particular
- Start from the very useful Guide to commutative diagrams packages (J.S. Milne 2005 –)
- diagrams.tex (Paul Taylor 1986 –), classic macros: much used and usually highly rated — though not so much by Milne — for drawing simple or very complicated commutative diagrams for category theory.
- tikz-cd (Florencio Neves 2012) This new package facilitates the creation of commutative diagrams in TikZ [see below] by providing a convenient set of macros and reasonable default settings. Seems worth checking out.
Packages for drawing diagrams more generally
- pgf and TikZ (Till Tantau 2005-: a general TeX macro package for generating graphics, with a user-friendly syntax layer called TikZ).
- XY-pic (Kristoffer Rose and Ross Moore 1991 – 2002; extremely powerful and versatile: there’s a chapter on this in The LaTeX Graphics Companion)
- Tree drawing in LaTeX (from LaTeX for Linguists)
- The semantics package supplies T-diagrams and other resources.
- Dednat (Eduardo Ochs, 2008) preprocesses proofs or diagrams written without markup.