J. chinensis procumbens 'Nana' (Japgarden Juniper, above), 1'hx10'w, gray-green foliage, compact spreader. 'Sargentii' is another chinensis, 1.5'hx9'w, light green foliage. It's branches have a whip-like appearance. Found by C.S. Sargent on the north island of Hokkaido, Japan. Dirr has over 90 cultivars listed under this species.
J. communis 'Effusa'(Common Juniper) 1'hx6'w, cold hardy to zone 2, turns a yellowish brown in cold months. Has a more open habit, showing off reddish stems. Native of northern regions of both the 'old and new world', and important in flavoring GIN.
J. conferta 'Blue Pacific', (Shore Juniper) 1'hx8'w, trailing, blue-green foliage. Even though the foliage is pretty prickly, it has a softer, hazier appearance. The branches are soft and flexible. Salt tolerant as well.
J. horizontalis 'Bar Harbor' (Creeping Juniper), 1'hx8'w, blue-gray foliage, purplish in winter, one of Peter's favorites as it's tips curl upwards making it look like waves. 'Wiltonii' (Blue Rug') is another horizontalis. A real ground hugger, it is possibly the lowest of the low growers. Also quite salt tolerant. 
J. squamata 'Blue Carpet' (Singleseed Juniper) 8"hx4-5'w, rich blue-gray-green foliage. Related to 'Blue Star' which has a taller more rounded habit (both have the bushy 'Meyerii ' as a relative). Purple cast to the foliage during colder months. Not as heat tolerant as the others mentioned.

Most of the above are low maintenance and trouble free, but can be susceptible to phomopsis (Juniper Blight) bagworms and spider mites. Also remember they don't like to sit in water. Good drainage is a must!  Once established they can tolerate dry soils. Come take a look at all the varieties we grow. Just look for the seas of green...and gray green, blue-green, yellow-tipped green, blue-gray-green, lime-green... you can't miss them! (Check them all out in M. Dirr's new 6thedition  of  The Manual of Woody Landscape Plants that just came in. It's a major revision of the last edition with 2000 new species and cultivars added).


FYI- Take a look at the Juniper Chart in our new 2010 catalog - page 42!

Also, Nearctica.com is an interesting site for detailed information of native conifers of North America. It's worth checking out.