Many of these vendors describe their stools as "Amish". I'm not sure if that means the design is somehow Amish, or if the stools are being made by Amish builders, or what. In any case, they're pretty handy, and a heckuva lot safer than that upside-down Home Depot bucket you've been climbing on to reach the top shelf where Grandma keeps her gin.
If you want to build one of these yourself, the following two pictures link to drawings that give the basic dimensions for the stool:
Almost any wood would work for this project, although it might be a good idea to use a hardwood dowel for the step pivot, even if the rest was made from softwood. Likewise, any number of joinery options could work as well.
While there's nothing magical about this particular design, it is very important to get the shape of the side pieces that hold up the steps correct, as well as the location of the pivot pin. If you don't, the step assembly might hit one of the stretchers when it shouldn't. Be sure to keep this in mind if you decide to modify the plans for some reason.