From the state of hidalgo Mexico, Mammillaria Humboldtii is a beautifully spined spherical cactus to 3 inches across. It can be solitary or clumping. In spring it produces a ring of pink or purple flowers near the top.
     I just got this one from B & B Cactus Farm with a few tiny offsets. I'll take them and pot them up when they get bigger. Below is a closeup of the beautiful spination of the plant.
     Update, April 2007. Flowers!

Closeup of spination

Flowering in April