There are going to be few perennials due to needing a slower metabolism in shade...less "calories" of sunlight...so shorter available energy pool to bloom. I'll suggest later season bloomers, there's a lot of spring stuff.
Rudbeckia triloba (blooming now...bloomed about July 1st...blooming till frost)..biennial. Reseeds easily.
Lobelia sylphitica (mid july till frost)
Mimulus species (if you have the environment for it), I have alata myself. July till September for mine.
Malvaviscus drummondii (if hardy in your area, you didn't say where you were...so I'm figuring on MY zone 6b) (July till frost)

That's what's blooming in my beds now...and all I can think of at the moment.
Clematis if you can get the top of the vine into the sun and leave roots in shade...will love it.

I have the following perennials that have a significant bloom period planted in shade:

Cimicifuga racemosa/black snakeroot
Tricyrtis hirta/toad lily
Dicentra exima/fernleaf bleeding heart
Chelone/turtlehead
Aquilegia/columbine
Cranesbill/perennial geranium 'Biokovo'
Hellebore/Lenten rose
Phlox divaricata/woodland phlox
Pulmonaria/lungwort

Be aware that, like all perennials, shade-loving plants have a finite bloom period each season unlike annuals.

The hands down winner is Corydalis "Lutea". It is in continuous bloom in even deep shade from spring till frost.

(The knock against this plant is that it seeds all over but it is so easy to pull it out that this isn't a concern to me.)

I second the corydalis lutea! Mine hasn't stopped blooming yet this year. I let it reseed and rarely have to pull it as it hasn't yet popped up anywhere that I really don't want it, although it is under the dwarf alberta spruce, and the back steps...both in mostly shade..

For me the winner is Dicentra eximia 'Luxuriant,' which also blooms May-Sept. Lots of catalogs and plant tag says a plant blooms that long, but this one really does.

I'd also add some coral bells to the mix. Some have very nice flowers and they bloom forever, but maybe they don't want deep shade.

You'd be better off with annuals if you want blooming in the shade all summer long.

Phlox 'Wanda' never stops blooming here in sun or shade.
Heuchera - many of mine bloom over and over. Don't need the blooms either.

We're still shooting blanks, possibly, as we don't know where OP is or how dense this shade is.

I'd also add some coral bells to the mix. Some have very nice flowers and they bloom forever.

Good reminder. My Heuchera "Lipstick" and "Havana" have been in flower for months and these flowers are nice.

This post was edited by rouge21 on Fri, Aug 8, 14 at 7:57

Definitely a good option mxk3. I have many yellow tuberous begonias that bloom all season in even deep shade.

Tradescantia sweet Kate has been a long bloomer for me this year-- I think the cooler temps have helped.

I'll third Corydalis lutea.

Apart from it's very free seeding, with it's fleshy root, it's very easy to pull out.

Getting rid of a Corydalis like C. solida (which grows from a corm) is much more difficult.

Corydalis lutea must be the leading candidate for longest blooming perennial where we live.

I haven't seen anyone mention of Astrantia, was one of the longest blooming carefree shade plants - if not allowed to get too dry - in my former garden. I did need to bait for slugs around it.

And it did self sow, but it bloomed early from the sowing and picking out those with the best coloring was easy, discarding the rest of the seedlings.

UPDATE:

Sunnyborders wrote:

Corydalis lutea must be the leading candidate for longest blooming perennial where we live.

And here it is now into the second week of NOVEMBER with several frosts and some light snow under our belt and still it is looking cheery:

Salvia farinacea blooms all season for me in shade and the plants stay upright with blueish foliage.

Cowpen Daisy blooms very well in shade with bright yellow sunflower type flowers but its an annual and its large so its best as a background plant.

Others are scarlet sage (S. coccinea) which naturalizes well.

Turk's Cap (Malvaviscus drummondii) blooms all summer and attracts hummingbirds like mad.

Salvia greggii will bloom as long as there is moisture but is heaviest in spring and fall.

Lamb's Ears has bright silver foliage for good color, shade is the only place it will do good here but I think it looks bad when blooming and worse afterwards.

Ruella is a good bloomer as long as the weather is warm but its seeds around A LOT. All of these have more flower power and less green than the plant pictured above.

Native types of echinacea, native yarrow and biennial rubeckias bloom well in shade but not for as long a period.

The shade I am describing for these plants is full bright shade by the way, some of it dappled. Dense dark shade is another matter and I don't think there is anything that blooms for a long period in that situation.

Knowing where the OP is located is going to make a difference here. Some of the suggestions (e.g. lamb's ear, salvia) need full sun to part sun at a minimum to do well where I am - the more sun the better for these two plants. Wish we could grow salvia in shade and lamb's ear - I'd love to have that combo in the shade beds! :0)

Probably so mxk3, the sun's rays are more intense in states further south so I imagine the shade would be brighter from more reflected light. Many plants that grow well in full sun where you are do better with some shade here, especially after 11:00 am or noon, otherwise they fry. I'd never considered this until I got on this forum and was amazed at the green lushness of the gardens further north in midsummer, its downright drastic compared to here. I had assumed it was just the heat but now I think its the light intensity of the rays of the sun too. That was when I noticed your mid summer is like our late spring going by bloom times and the overall condition of the plants in the photos.

Do shrubs count?
I have a (hardy) Fuchsia magellanica 'Versicolor' (such a pesky botanical name!) that's been blooming its heart out for weeks and weeks--still blooming at present. It has lost half of it's *versicolor* which has reverted to plain green. This plant gets only a little morning sun and although it's a common, old fashioned plant, I love it for its graceful growth habit, shade tolerance and lengthy bloom period.

I have a Choysia ternata next to the Fuchsia that also does incredibly well and is still blooming also. Great glossy green leaves and a sweetly scented white blossom--the twosome side by side reminds me of Christmas! Of course, Hydrangeas do well, too.

This post was edited by catkin on Thu, Nov 13, 14 at 0:41

Wish to H*** I could get Corydalis to even live in my garden. For some reason, I have never been able to get any of them to survive through even one year. I have tried at least a dozen times in different places and different depths of shade but no luck.

Hellebore, Pulmonaria, Epimedium, Brunnera, Asarum and Begonia grandis go wild But I just can't seem to get corydalis to live.

BingBingBing!!Rouge et al, youse is de WINNAH!! Corydalis lutea it is. In fact, I challenge ANYone in a z.5 garden to show me a true shade tolerant perennial with a longer bloom. Dicentra luxuriant made me pause for a minute, but I still think it loses to old CL. Wow, such a workhorse (a dainty workhorse-- is that an oxymoron or not!). And are we LUCKY or what? that it has a lovely lemon yellow flower and not taxi yellow!

geoforce, you ARE including corydalis LUTEA in your mention of corydalis failures? I have certainly lost the blue ones, but lutea is a whole different thing.
best,
mindy