It's as tall as me, and I wasn't sure what it was. A neighbor thought it was astilbe, and it does resemble astilbe a bit, but grows much taller. I was at Gethsemane Gardens yesterday, and I think I found it! Cimicifuga or actaea racemosa, aka snakeroot, bugbane, fairy candle, black cohosh. Awesome. The flowers are awfully pretty.
It has doubled in size since last year, and I'm not sure this is where I would have chosen to put it, but it is apparently something of a bear to move, with huge roots. So perhaps I will just leave it be. It seems happy, and is certainly dramatic in that corner of the garden. Sometimes, the best-laid plans of gardeners must give way to nature's happy accidents.