This is allium senescens. I bought it unlabeled at a plant sale four or five years ago and have since divided it up and placed it in every impossible spot in my yard–sunny spots in my super-sandy soil that are drained to a crisp by large trees.
No complaints, ever. This is not a bulbous allium, but a clump-former that's best divided with a shovel. The flowers are numerous but by no means spectacular, in a ghostly purple-white color. However, the strappy foliage and round shape of the plant are just nifty, offering crisp contrast to everything else in my borders.