Dianthus is one genus that will give you some reliable rebloom clear up until frost. {mouse-over links for image preview}

Dianthus 'Firewitch', 'Frosty Fire', and 'Neon Star' are three varieties that just keep blooming. There are also many new reblooming varieties that are coming out in the near future!
 

Dicentra is another genus with great reblooming ability.  'Bacchanal', 'King of Hearts', and 'Luxuriant' are all eximia cultivars that will keep blooming throughout the summer and fall. Fantastic plants for adding color to shady areas.
 

Iris germanica, or bearded Iris, are usually thought of as blooming just in the spring. However, there are some varieties that will, with the right conditions, rebloom in the fall.  These include 'Champagne Elegance' (soft apricot-buff), 'Immortality' (white), 'Pink Attraction', and 'Iris Summer Olympics' (yellow). Two dwarf varieties that also rebloom are 'Baby Blessed' (yellow), 'What Again' (yellow and blue bicolor).
 

For late summer color, it’s hard to beat the big, showy blooms of perennial Hibiscus. They come in many shades of red, pink, lavender, and white, with plain green foliage, bronze, or purple foliage. Hibiscus appreciate a little extra water, but they will also grow in average soil. New varieties are being developed with larger, ruffled blooms, better growth habits, and darker colors.  'Lord Baltimore' is a favorite for its large red blooms, 'Kopper King', for its bronze leaves and white blooms, and 'Lady Baltimore', for its beautiful pink blooms.  For 2010, watch for 'Summer Storm' (light pink flowers and purple foliage), and 'Cranberry Crush' (ruffled red blooms on a shorter plant), among other exciting new cultivars.

Sedums are probably the number one perennial for fall color. 'Autumn Joy' is the one most people know, however, there are many more varieties that add color to the late summer and fall landscape. 'Autumn Charm' is a new variety with green and gold variegated leaves. It is upright-growing, with light pink blooms. The bright foliage stands out all summer long.  'Angelina' is also new variety, with gold, needle-type leaves. It has a low, spreading habit. 'Autumn Fire' is similar to ‘Autumn Joy’, but has a much tighter growth habit. 'Carl' is an upright with very bright, pink flowers.

'Purple Emperor' is a purple-leaved variety that produces pink flowers in the fall. For 2010, watch for 'Hab Gray' an upright variety with gray leaves and yellow blooms, and 'Red Cauli' which has dark purple leaves topped by red flowers.  'Xenox' is another purple-leaved variety, but it has rose-pink blooms.