Clematis virginiana - Virgins Bower
Unlike the hybrid Clematis with large blooms, these have an abundance of small white flowers in the late summer to fall. One of the more attractive features of this plant is the seeds which make a white fuzz all over the plant, and is showier than the blooms. Will spread quickly but can be controlled, give it something to climb on.
Sun to Semi-Shade, Any Soil, Summer to Fall, Vine, Zones 4-9