What is this?

It's called Sedum spectabile. It attracts lots of insects when the fls are open. If you don't want it then I suppose you could call it a weed but if you do want it then it isn't a weed. It's up to you. It's a strong growing plant and seems to survive anywhere.

In the autumn the flower heads will change colour turning a wonderful reddish pink. You have a very healthy specimen there, wish mine looked as good :-)

If you want more of it, you can dig it up and divide it just as growth restarts in the spring

My sedum like this is tall and the flowers start out white (like shown) but then turn pink in Fall. You can also propogate by cutting off a stalk and sticking it in the ground. A friend told me about this, and I didn't believe her at first, but I tried it, and sure enough... the next Spring, I had a small plant growing up from where the stalk was.

Thanks to everyone for taking the time to answer my question. Zee