Free for the download, a new book called Trees for the Small Landscape by Charles Brun (Washington State University Extension, 2008) is well worth a look. 

Dr. Brun, a Washington State University horticultural adviser, spotlights 75 small-garden specialty trees, all with a mature diameter of 20 ft. or less. They include natives and exotics, deciduous and evergreens, conifers and broadleaved trees. I found lots of my own favorites there: sour gums and vine maples (great fall color!), stewartia (flowers and bark), fullmoon maple (pictured below).

He's also charted trees to that will grow under—not into—power lines, and made notes on best small trees for hot areas, alkaline soils, and wet sites. The focus is on the Pacific Northwest, but most of his suggestions will work over much of the country.