After a long period of quiet, I’ve added a bunch of new stuff to the xkcd store–three shirts, plus a bunch of other things. There’s a shirt based on the unproven Collatz Conjecture, a shirt about centrifuges, and this self-descriptive shirt:
In addition to the shirts, there’s a poster of my recent gigantic money chart (comic #980), complete with various minor updates and corrections. There are also a few other things I’ve added in the past few weeks, including some  stickers and a coffee mug for topology enthusiasts.
Note: We’ll be shipping stuff all through December. The holiday deadline for US orders is December 19th, so you should order before then if you want your stuff by Christmas.