Now is the time most people are planting Mums… they are hardy and will last if properly maintained (cut back in early July for late blooms) but soon we can plant pansies which will last until next summer… often blooming all winter and spring into July.
Some bulbs actually bloom in the fall, so this is a great time to plant them to get some color in late September and early October. Look for the autumn crocus, winter daffodil, Guernsey lily, saffron crocus, and some species of snowdrops. Usually, these bulbs spring to life soon after planting in late summer! For more bulb planting info, check out this article from the Dummies series.