Aloe variegata (Gonialoe)
Gonialoe variegata is one of the more cold hardy species. It can tolerate down to single digit temperatures and is a definate plant to try in a dry zone 6b. They survived 0F with no damage but the plant was dead at -17F. Regardless it is a very pretty species and great for containers.
I do not guarantee these in a zone 4/5/6/7 because everyones growing conditions are different but this would be a good trial plant to grow in your cold outdoor garden. Try to plant outdoor 90 days before 1st hard freeze.