The first plant and my all-time favorite Fall flower has to be the Aster. I have them tucked here and there in my gardens. Most of them are natives that I grew from seed. I do, however, have a hybrid-Aster "Purple Dome" which is a blooming star right now. I started with a small pot of it about 8 years ago and now from the same plant I have spread it around. It slowly expands every year and every year I take a piece or two off and plant it (or share it). It is a must have for your Fall garden.
Another excellent choice for your Fall garden is Eupatorium rugosum. I have both the native species and a hybrid called "Chocolate" which has darker foliage. Both bloom their heads off, usually lasting through until the middle of October, and like Asters, is a pollinator magnet. You should dead-head this one, though. It can be an aggressive self-seeder if left to it's own devices.
Toad lily (Tricyrtis) is a champ of a Fall plant. I have quite a few throughout my shade gardens both a white (no id) and the the purple spotted (Tricyrtis hirta). All of them I grew from seed. I had a couple of hybrids once-but they didn't make it through our brutal winter last year. I guess I will stick to seed grown varieties.
Anemones are another plant I would definitely recommend. They are easy to grow and very adaptable to any situation. I have some in a damp low area and some in a dry, tree root infested area. Both do well in my shady areas and are slowly spreading to fill the areas they are in. There are wonderful hybrids available but they are also especially easy from seed.
Actaea simplex "Hillside Black Beauty" is a star from Spring through Fall. When she isn't blooming, which is in October, her foliage is showy and really looks good behind my Hosta "Brother Stefan". The blooms are very fragrant-a far cry from my Actaea racemosa flowers which smell terrible. If you don't need the flowers add this one just for the foliage.
Helenium Autumnale-A wonderful native plant that should be planted just for the bees. It is pretty easy to grow as long as it has adequate moisture. I have mine growing in partial shade and it still rewards me with ample blooms every Fall.
Chelone lyonii or Turtlehead is a great Fall blooming native for those damp areas (although it will do fine in average soil with supplemental watering). It has lovely green foliage all Summer and when Fall approaches it blooms a vibrant pink that attracts droves of bees. I also have it's cousin Chelone obliqua (which blooms earlier) and enjoy having both in my garden.
Geranium "Rozanne" is not what you expect to see listed as a Fall bloomer. I had originally posted about this plant back in June when it first started to bloom. Well, it has been blooming since then. If you are looking to add a plant with a long bloom time I highly recommend you buy this plant. It is unsurpassed for it's long bloom time. As of the first week in October it shows no signs of giving up. "Rozanne" will probably bloom until the first frost. It also will have some Fall leaf color to go with those blooms when it gets a little colder.
An unusual plant you can add to your Fall blooming list is Gentiana andrewsii-Bottle Gentian. It will get plenty of attention from the bumblebees-since they are the only ones who will pollinate them. It will also get attention from visitors in your garden asking "what is that plant" since it is a unique plant to have in your garden. I also have Gentiana lutea which I also recommend-it blooms a bit earlier in the season.
The very last plant to bloom in the garden is Aconitum carmichaelii. This one starts in early October and will bloom until the first hard freeze. It is well worth the space having this plant in your garden.