Does it get full sun? Clethra would work; there are several cultivars, be sure not to get the dwarf one if you want 8 feet tall. Viburnum nudum ('Winterthur' is one cultivar) tolerates wet also.

Summer flowering? that can be a little tough. Most shrubs are spring flowering. There are some very large growing perennials that adore wetter areas and flower more towards the sumer, but they tend to like at least a little shade.

I'm really not sure how althea (rose of sharon) would do for you. It meets the requirement of being a summer flowering shrub af 8' in stature, but not sure how it would do with the wet feet.

Otherwise:

Numerous viburnum species, and a great plethora of varieties.

shrub dogwood, a number of varieties.

winterberry holly, again, several to choose from.

willows, such as Hakuro Nishiki, arctic blue leaf, Flame, and others.

Itea (sweetspire), but not the compact varieties.

My best advice is to find a local nursery and see what they have to offer and suggest.

All clethras will tolerate excessive moisture and even prosper in such conditions.
By planting different cultivars you may create non-stop blooms from July (C. alnifolia species) till September ('September Beauty').

I have a similar situation in my yard, and what is there is a cherry tree - supposedly high on the list of things that don't like standing water. This is a very different site than my swamp, which never really dries out.

The tricky part about temporarily wet places, is that often they are caused by hardpan soils that when they do dry out, dry out very hard. So the plant stress isn't from water, but from the opposite.

It will be best to fill, if possible, the low areas causing the sporatic standing water area. Spread good draining sandy loam top soil before spending lots of time, effort, and money trying to get anything to grow there. You might also need to excavate enough down slope or shallow trenches for any surface water, which still runs off to that area, to drain away.

In silty loam and/or high clay content soils, hardpan sounds like an appropriate description for low areas which stay wet for a while after soaking rain events, but which often also become bone dry between such rain events.

I own a lot with the same situation as mickeddie describes. Even the Heritage River Birch tree I planted there has struggled. When planting that tree I found the silty loam soil to be hard compacted from frequent standing water episodes. The ground was so hard that digging a proper planting hole and installing the stakes for that 10 gallon potted sized tree nearly wore me out.

In the 2005 summer through 2006 winter dry periods, the birch tree dropped its leaves and died back almost to the ground. Toward the end of last year's summer drought period I had about 12 dump truck loads of a good sandy loam top soil spread to fill the low areas causing the water to settle in that area. I thought the birch tree would surely die when the next heavy rain event washed almost 6 inches of that fresh non-compacted soil over the root ball of my struggling birch tree.

Surprisingly, now the birch tree is recovering nicely. The tree has spread it's roots into that washed in sandy loam top soil and is doing so well; it appears promising that the birch will survive to become a beautiful tree.

Standing water no longer lingers around that birch tree, but the soil does remain damp a few days longer than the rest of the lot.

With an organic mulch, the moisture should stay enough to keep the soil damp and wetland plants happy. In addition to clethra, which is late summer blooming with pink or white flowers, Rhododendron viscosum (swamp azalea) blooms white or pink in early summer and has nice fall color. Rosa palustis (swamp rose) has single pink flowers in early summer and red hips in the fall. It has the added benefit (since you want to keep out trespassers) of having thorns.

There are several damp loving shrubs that don't have summer flowers, but do provide summer interest along with something of interest for at least 2 other seasons. Some of the shrubby dogwoods, while they aren't summer blooming, have variegated foliage and bright red or yellow winter stems as well as small spring flowers and berries that the birds love. Blueberries have white spring flowers, summer berries, and bright red and orange fall colors. Aronia has white spring flowers, summer fruit, and brilliant fall colors. Two other moisture-tolerant rhododendrons have spring flowers, R. atlanticum and R. periclymenoides (also called nudiflorum.)

Clematis can take it a bit wet. You can grow a clematis on a variegated dogwood like this:

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I second the vote for Clethra. It likes wet, most varieties sucker and fill in to make a better hedge and the summer bloom smells wonderful.

Spiraeas are moisture-loving, a native species forms thickets in wet places here, which have sometimes been called "prairies".

Spiraea japonica is common in nurseries, flowers in summer. Many cultivars to choose from.