Found this on digg. Built one this morning out of an Amazon box. It works pretty well, although I like my tshirts trifolded vertically and this only does bifold. Ah well, maybe it’s time for me to switch that up, if only so that I can use my new machine.
UPDATE: For people who can’t use cardboard for whatever reason (lack of material, religious beliefs, etc), you can use this Japanese ninja method.
I used to have a t-shirt folding machine, but she made me move out and go to college.