links for 2008-02-23

From A neighborhood of infinity: What is topology?, attempting to provide some intuition for the standard axioms that define a topology in terms of "observable sets" (which are just open sets, with different intuition attached to them!)

Also, on a totally unrelated note: John Baez has online notes from the course he gave in Fall '03, Winter '04, Spring '04. The winter notes are particularly interesting to me, because they combine combinatorics (which I know well) and a lot of category-theoretic notions that I'm not too familiar with.