Nearly every garden design book I've read mentions Verbena bonariensis. This lovely perennial blooms the first year from seed with clusters of rosy-purple flowers that attract butterflies and hummingbirds.
Verbena bonariensis is a nice companion plant for roses, ornamental grasses, and tall perennials like trumpet lilies.
Verbena bonariensis is hardy in zones 7-10, but it often overwinters in my zone 5 garden. Grow it as a self-sowing annual in colder regions. It is very vigorous and may be considered invasive, especially in warmer zones. Also known as Purple Top Vervain.