Contoneaster, but flowers are very small and do not last long. Periwinkle, i do know that this is a shade tolerent cover but seem to think its also sun tolerent also(not sure) Hypercum ST. johns wort has yellow flowers all summer....
We use Green Liriope( Big Blue) ,has a large blue flower that flowers all summer. The other is Bar harbor or old Gold ( junipers).They don't flower but the color when mixing types makes a good backdrop.
Not sure whether your idea of low-growing is what shrub people think of as low-growing or what perennial people think of as low-growing. There's about two feet of difference. If you mean under 12", I'd suggest small-flowered pansies. Here (Zone 3) they come out of the snow with green leaves and buds (from the past season). They bloom like crazy, crossbreed among themselves, and self-sow freely. I have large established areas that are in bloom from early spring to snow cover. They can be mowed (blade height 5") if they get too straggly.
So what do you suggest for a hillside with absolutely no shade. I have one property that has a poorly wood-chipped hill that is too steep to mow, so I end up trimming the weeds out of it. It takes forever, I've suggested they invest in sometype of ground cover. Is there anything I can plant now and have a decent stand of through this summer?? I really don't want to have to f with that hill again all year.