Purple larkspur.  Can't do without it! It will seed down very well too. Just wait until the seeds have turned black and drop them where you need filler next Spring. I don't even cover the seed.
Cold tolerant, larkspur can be planted in fall in even the coldest corners of the world. Speed up germination by chilling seed in a refrigerator or freezer for a week before sowing.
Rocket Larkspur   Delphinium ajacis (Ranunculaceae)    A winter annual, native to southern Europe, it has naturalized throughout the United States. Larkspurs are distinguished by a backward projecting spur, formed by the upper petal of the flower. Tightly compact blossoms are arranged on spikes in shades of pink, white, and deep blue. Prefers full sun to partial shade in very well-drained soils.
~~Delphinium 'Fenella' | Garden Buddies forum~~
Candle Larkspur, Delphinium 'Centurion Sky Blue' (Delphinium elatum)
Growing larkspur flowers provides tall, early season color in the spring landscape. Once you learn how to grow larkspur, you will likely include them in the garden year after year. Read here for more info.

Growing Larkspur Flowers – Information On When To Plant Larkspurs

July Birth Flower : Larkspur
Larkspur flower
Top 10 Easy Perennial Plants to Grow From Seed

Top 10 Easy Perennial Plants to Grow From Seed