It would be helpful to know either your town/location or Sunset zone, as recommendations will vary depending on your local climate. Some perennials I like using for the extraordinary length of bloom season would include many Mexican cloudforest Salvia species that are fall and winter blooming but can also get shrub sized and may not be that freeze tolerant. Also more common plants such as Verbena bonariensis, Calandrinia grandiflora, Erysimum linifolium 'Bowle's Mauve', Justicia brandegeana, Plectranthus zuluensis, Justicia carnea.

In many areas of SoCal you need to plan farther ahead fall color. I find many of my summer flowers will bloom into early December with deadheading (Hemerocalis 'Russian Rhapsody', Rudbeckia 'Indian Summer', Gaillardia, to name a few). Fall becomes a plan for December-March flowers, and the inserting the annual-grown perennials that go until I yank their tired little bodies out of the soil (dianthus, pansies). Freesias are a great one to plant, if you are not in too cold an area. Best deal I know of is a bag for $12 from CostCo in the fall; they will bloom every spring and multiply, so you really only need to purchase them once. Kalanchoe Mirabella can provide red flowers January-April every year. Bletilla is another amazing performer that blooms from about February-May for me. They aren't invasive, but they do grow quickly and can be divided. I started out with 2 small rhizomes, and now there are so many I used them en mass in a sidewalk strip.
As Bahia said, it depends upon where you live. Are you looking for sun or shade plants?

Aloes and Camellias are the stars of my winter garden. The Aloes attract hummingbirds, too.

Don't restrict yourself to perennials! What about shrubs and trees? Not necessarily a garden full, but a few choice specimens chosen for fall and winter interest.

Sara

Here is a link that might be useful: Form and Foliage: Bleak Midwinter

It's best not to plant much in the heat of the summer in southern California- we wait until October so that our plants have a better chance of surviving.

Chrysanthemums bloom in fall, and you can get them in the fall already in bloom.
Brugmansias and cannas bloom in the fall and into the winter if there is no frost. So do some daylilies. Roses bloom into January. Reblooming tall bearded irises bloom in the fall and winter all the way through to March (Violet Turner, Total Recall, Mariposa Autumn, and Recurring Dream are my best).
My spireas, calla lilies, and azaleas bloom in March.
Renee