One of my very favorite topics, thanks for the A2A. This list is an off-the-cuff suggestion of hardy, perennial clumps/bushes that are both pretty and drought-tolerant, designed for swales by gardeners in Denver, Colorado.

• Yarrow - hundreds of varieties for colour/density.
  
• Russian Sage - gets really big and pretty, smells like Lavendar.
• Big Sage - naturally pockets hillsides around town.
• Most Salvias - bushing kind, not culinary herb-kind.
• Currants - less drought-tolerant, fruiting kinds.
• Hyssops - several 'naturalized' varieties.
  
• Choke cherries - will spread, needs space.
• Grasses - pampas varieties.
• Oregon Grape/Mountain Hollies (tolerant of partial shade, too).
• Desert Willow - given enough time, will become a centerpiece to any garden. I cannot describe how beautiful this knotty, bushy shrub is when it blooms. I've found a photo:

Personal Experience w/Swale-Denizens, Medicinal Herbs:

• Horehound/Motherwort mix, these two seem complementary companion plants that have yet to wilt in a far-from-the-hose patch.
• Sempervivum varieties - green to red to spider-silk kinds have merged well into the rock-wall-build-up.
• Yucca(s) - small, sturdy, basal branching. Broadleaf/narrowleaf varieties.
  
• Feverfew - two year-old bush of small, white flowers.
• Strawberries - like to spread, hard to hem in - less drought-tolerant.
• Valerian - hasn't wilted, not typically thought of as drought-tolerant.
• Fennel - nothin' has killed it, including neglect.
• Calendula - a.k.a. 'Pot Marigold.' Tall, showy yellow-to-red Compositae/Daisy-Like... not the ones from South Africa.

Ground Covers:

• Clover.
• Menthe spp.
• Cinquefoil - hairy, shrubby, etc.
• Bear Berry Varieties - Manzanilla or Arctostaphylos, hybridized.

Unfortunately, I do not have the space for real trees, but this next year I anticipate putting in Raspberries at the base of my steep-slope. While a bit thirstier, I've seen patches that have excellent results and the slope still has a lot of run-off. It's a work-in-progress. One of these years, I'll take some photos.

I hope that this helps!