This page is ordered by age, and only includes machines powered by an electric motor. A separate pictorial index includes all the machines I've ever owned.
Electric machines had appeared in very small numbers, earlier, but they started coming on the market to stay in the late 1910s. Many manufacturers used add-on motors at first, often purchased from Hamilton Beach. Singer's first electric-only machine, the 101, came out in 1920.
Click any image here to see its blog entry, and more pictures.
The main blog page is kept in the order the machines joined my collection (and also has other sewing machine related entries).