Euphorbia, "snow on the mountain".

I've got in my garden, too, as a volunteer. It comes back every year.

Some people (me!) are highly allergic to euphorbia sap.

Here is a link that might be useful: http://www.plantoftheweek.org/week274.shtml

I'll have to remember that next spring. I definitely want some of that in my garden, it's beautiful!