Native to a large swath of the U.S. and Canada, Escobaria vivipara is a clumping globular cactus with heads up to 3 inches around. I've read that colonies of as many as 200 heads can form. This must be a fantastic sight when the clump is flowering. Flowers are pink to magenta and about an inch across. This plant is known to be very cold hardy, withstanding temps to 30 below while covered in snow.
    I got this one in a trade with Bill Williams of Kansas City. I know Bill from the forum at Cactiguide.com. He's a real fan of cold hardy cacti. He was also kind enough to donate the flower pictures below of E vivipara var. rosea. Thanks Bill :-)
Unfortunately the plant above got severely dehydrated over the winter and I can't seem to bring it back. Fortunately I was able to find another variety at Bachs. The later pictures are of Escobaria vivapara var. kaibabensis.

Flowers by Bill and my new plant.

A nice clump flowering	A closer view	Escobaria vivapara var. kaibabensis	Flowering in March
Three at once in June