Never Say Die---Mystery Cactus

When we bought this house a few years ago there was a lonely cactus growing amongst the heavy weeds along the house. This little cactus keeps appearing year after year. It was even pulled out and tossed frivilously back on the ground. It managed to re-root itself and continue to thrive. I didn't even know that a cactus could be hardy in zone 6! See what I know? I have no idea what kind of cactus it. I never bothered to figure it out, but if you have any idea, let me know. It's one tough little plant that refuses to die. I'm glad though, because this little oddity has grown on me.