I plan to buy more phlox (David and Volcano) next year. My David is still blooming beautifully and has been since July. I cut some of it back and in a short while it's covered with bloom again. (The 17 inches of rain we had in Aug helped.) Agashache Blue fortune--not the most wonderful color but it just doesn't quit blooming. Bees and butterflies all over it.
Roses of course bloom from May til Oct.

There was a really detailed post on this a few weeks ago... and several others that have been archived here and can be found via the link below.

Here is a link that might be useful: GardenWeb Search for Long Bloom

white clover is very long blooming - ok just kidding. I like a succession of the different kinds of lilies. Also, Clematis Betty Corning is one of the longest blooming plants in my whole garden - still blooming and I think it started in May this year. Self seeding Nicotiana never seems to quit - still going strong everywhere, and so is Dianthus barbatus. Monarda is one of the longest perennials. Other than that, in part shade, everything has its season.

I agree w/the Agastache, and will add Moonbeam coreopsis (my favorite perennial) and Lamium. Dianthus usually reblooms a few times for me if cut back, as with perennial geraniums.

Jen

The longest-blooming perennial I ever had was delosperma. It bloomed from May till the frost got it in November. The only problem is I have trouble getting it to overwinter. I tried three years running and then gave up, only to have a reseeder pop up for me! I had cooperi Starburst. Maybe another variety would be hardier....

Pretty little plant, takes neglect and poor soil, and blooms for six months. If you can get it to overwinter, what's not to love?

:)
Dee

Long blooming perennials (three months plus) here include: coreopsis Moonbeam, echinacea white swan, echinacea bravado, gaillardia Arizona Sun, gaillardia Goblin, Rose of Sharon, buddleia, rudbeckia. Good luck and enjoy.

Here are the long bloomers in my garden
Threadleaf Coreopsis - I think it is moonbeam
False Sunflower - abundant bloomer from early july until the frost
Golden Showers roses - June until frost
Rose of Sharon - lots of flowers through august
Echinacea - I have a yellow variety, i little shorter than the false sunflower
Black Eyed Susans - same as above
Daylilies - my stela d'oro bloomed late June until now
Lamium bloomed all summer
Geraniums - bigfoot and cranesbill - not a big show but bloomed from May until labor day

I just have the annual (to me) Scabiosa atropurpurea; they keep on keepin' on too!!

Coreopsis lanceolata
Echinacea spp.
Rudbeckia hirta
Heliopsis (False Sunflower)
Blue Flax (cut back for another couple months of blooming too)
Feverfew 'Flore Pleno'
Gaillardia aristata 'Burgundy' and common species Gaillardia aristata

Runner up's.....

Lavender (if I choose not to harvest....cut back for re-bloom)
Hollyhock (mine are perennial....cut back early bef. first flowers open if you want a bushier shorter plant with more show first time around. Cut back again pretty low for a fall display too :D )
Maltese Cross
Sweet William (they are perennial if not allowed to set seeds!)

Are we talking with or without deadheading??