Carolina Allspice, Clethra alnifolia

Clethra alnifolia, Comptonia peregrina, Solidago odora, and Anaphalis margaritacea.

Asclepias syriaca, commonly called common milkweed - summer flowers, nice sweet scent as you walk near them to enjoy the butterflies. Other milkweeds may be prettier but none smell as nicely

Rhododendron viscosum, the swamp azalea -- early spring flowers, scent wafts a long way, very nice if you have the acid soil it requires

Magnolia virginiana, commonly known as sweetbay magnolia - a nice small tree, lemony scented flowers

Calycanthus floridus, commonly known as Sweetshrub -- some plants give their scent easily, others you have to bury your nose in the flower to smell it at all, so if possible buy this one while it has blooms so you can ensure you get a good one

Also beware of named varieties of clethra -- I bought a variagated clethra and it had zero scent, ever. It put me off clethras ... but probably it was the variety. Anyhow, personally I'd want to smell the plant before I tried another one.

Ken Druse just put out a new book about fragrance the garden, "The Scentual Garden."

I have Clethra 'ruby spice' and it smells incredible. The normal white wild types are fragrant as well.