Barleria cristata (Philippine Violet)

Barleria cristata is a carefree shrub that blooms in both spring and fall.  The 1.5-inch diameter, purple flowers nearly cover the plant when in full bloom.  Plants grow 4-6 foot tall and wide.  Tall stems tend to arch over, so keep them pruned shorter if you want a more compact look.

There is also a white flowered variety, and I used to have a pink one too, but it died out one winter.
Flowering will be best in full sun, but I have some in a considerable amount of shade that still produce some flowers.  The white-flowered variety especially, really stands out in the shade.

There is a variegated-leaf form that has spectacular foliage but few blooms.  This one is also good to brighten shady spots.

Barleria easily spreads by seed so it is preferred for mixed borders or as a hedgerow, where stray seedlings won't be a nuisance.
Barleria cristata, also known as Philippine Violet, is a native of India and southeast Asia, and is recommended for USDA Zones 9-11.  There may be some dieback on exposed plants during severe winters, but mine fared well at 26°F last winter.  Even in colder locations, the plant could be grown as a fall-flowering perennial.

An extract of Barleria leaves has been used in traditional medicines for the treatment of anemia, toothache, and inflammation.  Scientific studies have confirmed that it has strong anti-inflammatory properties.