I have a 5 acre parcel of land and just excavated it, leaving just large cedars and firs. I am trying to find a low ground cover that's pretty dense and can cover at least an acre of hillside that can't be mowed. Any help would be appreciated.
This answer assumes your hill lies on the West side of the Cascade Mountains. Hillside plantings for the Okanagan or the more arid and cold areas of Washington, Oregon and B.C. would look quite different.
Spreading cotoneaster (Cotoneaster sp.) is very common and successful plant for hillsides in the Pacific Northwest and, as Bamboo notes, C. procumbens 'Queen of Carpets' is an excellent choice that is low (< 1'), wide spreading (6-8') and quite dense. Other Cotoneasters well adapted to this area include bearberry cotoneaster (C. dammeri)(1' x 6'), gray-leaf cotoneaster (C. glaucophyllus)(3' x 6'), willow-leaf cotoneaster (C. salicifolius)(2' x 6'). For a large hillside, the taller varieties (2-3') may be more effective as they are better weed suppressors.
St Johnswort (Hypericum calycinum) is another commonly used hillside plant, 1.5 feet tall and 4-5' wide, all summer flowering and very tough, but can look a bit tired by the end of our typically rainless summers.
A Seattle hillside of Hypericum calycinum in early September
The Pacific Northwest is a weird climatic hybrid between wet maritime England, and the dry summer Mediterranean coast, so there are lots of other options. Any good choice for a dry hillside in the Northeastern or Midwestern U.S. will work for you. Hence junipers (Juniperus sp.)(2' x 6'+, but slow), Gro-low fragrant sumac (Rhus aromatica 'Gro-Low')(2' x 6'+), or bush honeysuckle (Diervilla sp.)(1.5' x 6') are good choices for your hill.
Diervilla sp. planting near Everett, WA
If the slope is sunny, warm and sandy, you can also use plants more typical of California such as rock rose (Cistus x hybridus, C. purpureus and others)(1-3' x 6'+) or California lilac (Cenoanthus gloriosa), but, after these plants are established, they should get NO supplemental water during the summer.
Cistus sp. planting near Everett, WA
Alternatively, there are number of attractive native groundcovers. bearberry (Actostaphyllos uva-ursi)(0.5' x 4') is an attractive mat-forming broadleaf evergreen that is found native growing on sunny, sandy, acidic locations. For shadier spots, creeping Oregon grape (Mahonia repens)(1' x 6') and salal (Gautheria shalllon)(In sun, 1' x 6'+, in shade 4' x 6'+, suckering great distances) are better choices. Both are evergreen, adapted to dry unirrigated sites, and grow in full sun as well as shade.
A Seattle planting of Mahonia repens with Actostaphyllos uva-ursi behind
You can, of course, plant several of these. Since you have over an acre to work with, it would seem a shame to use only one plant.
As a final option, you can leave the slope as it is or plant any of the above but fail to consistently weed. In five to ten years, Himalayan blackberry will own your hillside. The birds will be pleased, but your neighbors might not be.
Himalayan blackberry near Everett,WA
Cotoneaster Queen of Carpets, height up to a foot, spread up to 6 feet or more, or C. microphyllus, which forms more of a hummock reaching about 2 feet as it matures, spread up to 4 feet, sometimes more. Both provide evergreen ground cover, though in severe winters, may be semi evergreen; takes a while to achieve full spread (probably around 8 years). Tiny flowers much loved by bees, red berries in fall on both.
Update: Just thought of another one - dwarf Bamboo, the Pleioblastus varieties such as P. pygmaeus, which spreads up to 6 feet because its quite invasive, gets about 10 inches high. Note that the variegated ones (P. variegatus, P. auricomis) do not have such a wide spread.