A top pick for xeriscape sights is the Washington Native Plant Society. It includes all kinds of resources from how to plan your xeriscape to native plant and seed sources.
Here are some WNPS resources to check out:
They have extensive ecosystem information, from the wet San Juan and Olympic Rain Shadow ecosystem to the arid Shrubsteppe ecosystem that prevails east of the cascades. According to the site: "shrub-steppe communities are amazingly complex...less than 40% of the original area covered by shrub steppe remains unconverted in Washington."
Our Northwest folks especially love the Native Plant Gallery Index. It contains over 150 detailed discriptions of native trees, grasses and shrubs.
Pictured from the index is a sand-dwelling wallflower that grows in open ridges, talus slopes and rock crevasses of Washington, Oregon and Idaho.