There are so many plants that would work for your zone. I'm zone 5 and grow many varieties of sedums, semperviviums (hens and chicks), dianthus, perennial geraniums to name a few in my rock gardens.

Sedums are a good choice as they are very hardy, have interesting foliage colors and textures and have flowers to boot. There are sedum cultivars that grow tall such as the popular Sedum 'Autumn Joy' and creeping varieties that grow from 1" to 4" tall. Most are pretty well behaved in the garden not spreading too aggressively. For smaller gardens you may want to avoid cultivars such as Sedum 'John Creech' or Sedum Acre as these can spread more than you may want. For creeping varieties, check out Sedum spurium 'Dragon's Blood' or 'Fuldaglut', kamtschaticum 'Variegata'. Sedum takesimense is one of my favorite sedums forming a nice neat mound with long lasting yellow flowers.

Semperviviums or 'hens and chicks' are popular as they are drought tolerant plants which can stand poor soil. They come in many different colors (green, purple, silver, mixed, cobbed webbed).

Dianthus are great plants for the sunny rock garden. These plants have grey-green or blue foliage with pink, red or white flowers that bloom in early summer. You can grow some of the dianthus species from seed. Check out 'The Fragrant Path' seed catalog and you can order many dianthus for $1 a pack from this catalog. Be sure to get the perennial dianthus and not the dianthus sold as annuals.

Lastly my suggestion is for perennial geraniums. Not to be confused with the annual geraniums found just about everywhere. Perennial geraniums have small flowers usually white, pink, blue or purple and interesting leaf texture. Many of the leaves turn red in the fall when the seasons change. Two I'd recommend are Geranium sanguineum 'Striatum' or Geranium 'Rozanne' developed by Blooms of Bressingham that blooms for an extended period of time.

Here is a link that might be useful: Fragrant Path Seeds (at GardenWatchdog)