Scabiosa (I like "Butterfly Blue, personally) is one of my favorites. If you deadhead it pretty regularly, it will just bloom and bloom and bloom...and, oh by the way...BLOOM. If you DON'T deadhead it...it's still going to bloom, just not as much. In fact, it has even bloomed a little bit during the WINTER where I live!

You aren't excluding Cannas? They start early and finish late. Dianthus (Fire Witch) seem to bloom for an extended period then recede a bit and bloom again. Hybiscus syriacus, Rose of Sharon bloom continuously during the season but, are deciduous. Ice Plant blooms all season long. I like these giant Black Eyed Susan looking things. I have heard they were Rudbeckia but, the basal leaves are large even for R. hirta and the flower production is non-stop so, I am unsure as to actual species. I first purchased these at Home Depot for a job, just to fill in a small empty space. They were listed as an annual for $4.75. After the first year they reproduced several more plants from seeds and, the originals returned as well so, I brought a couple home. They have been blooming steadily since early June. The flowers are about five inches across and the plants get about two feet tall.

Goldsturm Rudbeckia looks really good, just about all season long. In my case, after about 2 years, my local deer herd decided to eat it to the ground. Repeatedly.

Ditto on the Goldsturm Rudbeckia. Also, my phlox carolina is still flowering (since May). So is lonicera sempervirens, though its really only showy in spring. A rare thing in natives, which is what I tend to have, other than roses and veggies.

There is a thread on the Cottage Garden forum with some good suggestions too...

Here is a link that might be useful: CG thread