Dietes grandiflora (large wild iris) is a rhizomatous perennial from southern Africa with long, rigid, sword-like green leaves belonging to the Iridaceae family (ht to 1m). There is a very similar species known as Dietes iridoides, and I am not really sure how to tell them apart.
The blooms are white marked with yellow and violet and appear in flushes in spring and summer. The flowers are followed by large seedpods and they are very prone to self-seeding. Note that they are regarded as a weed in some parts of Australia. They will grow in full shade (flowering less) to full sun, where they will bloom more profusely. The plant is are drought and frost hardy. I use it in spots where basically nothing else will grow, such as along my lengthy driveway, where they have formed a solid band of foliage.
Postscript: over the years I have become disillusioned with the plant. It is certainly tough and will grow in shade, but its vices outweighed its virtues, especially its self-seeding habits. I also found it didn't flower very long. I wouldn't recommend it these days. However, two cultivars are the exception to this: 'White Tiger', which has very attractive white-variegated leaves and the same white flowers as the species, and the cultivar 'Grand Star', which looks similar to the species: neither of these appears to self-seed.