Would these be large enough?
-Sedum 'Autumn Joy'
-Rudbeckia 'Goldsturm'
-Rubbeckia nitida
-Rudbeckia triloba (biennial)
-Chrysanthemum - the large flower series such as Football Mums from Bluestone
-Perennial sunflowers such as Helianthus maximillianii
-Hydrangea arboescens 'Annabelle' ---not really a perennial but acts like one. Has very large leaves & flowerheads. Very attractive
-Large hosta such as 'Elegens' and plantaginea. Plantaginea blooms in August. Very, very fragrant large white flowers....
I also love Ornamental grass. They are most spectacular in fall.

I forgot to ask; are you looking for bulbs?
If so, there is Lilium speciosum rubrum that blooms in August...normally..that is. But this year everything is a few weeks early, so I don't know if the lily will bloom before August or not.

Colchicum will bloom in September-October. Licorice squamigera will bloom in August.

I know you specified perennials but have you considered dahlias? Yes they have to be lifted and stored for winter (unless you don't mind replanting each spring) but they give one heck of a flower display from last summer through 1st frost. The "dinner plate" varieties have massive blooms and some of the non dinner plate varieties have good sized blooms as well.

Just a thought. :)

Here's Dahlia Otto's Thrill Sept 5, 2006

{{gwi:20708}}

to pitim...still thinking on the SEDUMS. Been looking at them as a possible for years. My Rudbecia bloom in July. I dont think the CHRYS will be winter hardy but will check on those. Helianthus...will get some, thanks. likewise the Hosta.
to whynotmi...ya know, I had Dahlia before. Loved the flowers. I dug and stored per instructions and they never regrew. tried a second time and same result. I think my prob with Dahlia is that the only warm place I have to store them is here in the house where it is 70+ all winter. No garage, no basement to keep at 50-60.

Pink Turtlehead (chelone) blooms in August for me, as does Helianthus Lemon Queen. The turtles are small in size but unusual & quite noticeable. The crisp green foliage stays nice all season long. Lemon Queen has lemon yellow daisy flowers on 6-8 ft plants. Lobelia Cardinalis starts to blooms in late July here and is a nice spiky contrast to all the daisy flowers at this time. Mine get a good 5 to 6 ft tall. All these need a bit of moisture. These are about the only ones I grow that are supposedly hardy in zone 4 according to Dave's.

Other possibilities, if you push the zone, would be Japanese Anemone, Toad lilies, Obedient plant (mine are starting just about now, and Hibiscus moscheutos (huge flowers on 6-7 ft plants (buds but no flowers for a couple more weeks).

Some of the Miscanthus grasses that I grow do not bloom well for me unless our fall is long, as sometimes winter comes all of a sudden here. Strictus is one them that only blooms some years, but I actually prefer that one without the blooms. Graziella is an early blooming cultivar (starts by late August, sometimes early Sept) , which doesn't have much going for it except for blooms, but they are awfully nice & it always blooms for me. The Feather Reed (Calamagrostis) grasses are already in bloom here and the panicums are just now starting. These are all hardy in zone 4. These don't just add color to gardens, but movement as well. Wouldn't be without them.

If you buy your dahlias at Costco for example they are only about $1 each - they usually come in stock in early March so you can start them indoors. Treat them as annuals and let them rot in place. You don't need that many to make a nice impact. I have decided that nothing beats dahlias for the end of the garden year. The white flower I posted above is also a non-hardy bulb but you've never smelled anything so fragrant - Acidanthera. I can hardly believe it but I grew Dianthus barbatus from seed last year and they started blooming the 2nd week in May and are still blooming. Its July 25 and my first Lilium Black Beauty opened today along with the first Casa Blanca. The species lilies and martagon are weeks away.

Other perennials for August
Lobelia Cardinalis - red
Platycodon
Rudbeckia and Heliopsis
Aster Honeysong Pink and Alma Potschke - you won't convince me this isn't enough color - they are gigantic and totally eye catching.
Boltonia
Hardy mums Clara Curtis and Mary Stoker - daisy type
Aconitum, Chelone Hot Lips and Tricyrtis
Dahlias
If you cut your Becky and Alaska Shasta daisies back hard and re-fertilize when they are finished in July, you may get a nice re-flush of blooms late - ditto for Monarda, Phlox
Anemone Robustissima - pale pink
Astilbe pumila Chinensis
Heuchera Firefly and some of the other plain green foliage with red flowers
Shrub Hypericum Albury Purple - still blooming today and totally covered in red berries with purple tipped foliage
Salvias and Penstemons
Lavender - with protection or replace
Variegated sages and coleus - foliage color and purple flowers

September
Hybrid mums till hard frost
Dahlias - till hard frost
Japanese Anemone Pamina, Honorine Jobert, Andrea Atkinson
Impatiens and bedding begonias till the bitter end
Hosta Honeybells and Plantaginea - the 2 fragrant whites