I've had this weed (well, not this particular specimen, but you know what I mean) in my garden for forever, and have never bothered to find out just what it is. Grows tall and lush, quite pretty really. I don't recall seeing flowers, but it must flower for it to reseed...

Agree it is very pretty! Sorry I don't know what it is.

I'm going to guess Chamberbitter, aWeed in these here parts.

Looks like a reasonable suggestion, but from Chamber Bitter (Phyllanthus urinaria) I gather chamberbitter is only supposed to get 14-18" tall. My weed is at least three feet tall, so I suspect it's something different.

Possibly Illinois Bundleflower (Desmanthus illinoensis)

It does look like Illinois bundleflower. If so, it will produce blooms, but they aren't particularly noticeable (little white pom poms). The mature seedpods are hard to miss, though.

Hmm, in that case it would not be a weed, but an on-purpose plant. I did grow some of those last year, but thought they hadn't survived. I think you're right, that's probably it.