This was an unnamed cactus when I bought it- but I believe it's a Mammillaria nana ssp. duwei. A lovely little fluffy globe about 3cm in diameter. Doesn't grow much, but always has a flower or two to show. So pretty!
The genus Mammillaria has so many intriguing and just plain beautiful species, (about 250, I think); a cactus fanatic could just collect them and nothing else and be content. I particularly like the dwarf species such as this one. They hail from Mexico and the SW USA and are pretty easy to grow.