Northern Alligators, and Southern Alligators. Southerns tend to be larger and usually a tan color. Northerns more of a chocolate color (at least in the Willamette Valley).
A skink should be easy to differentiate by the blue tail (if young), or faded blue (if adult) and the fact that their scales are completely smooth, whereas alligators have keeled, or rough scales. Much more colorful 'striped' dorsal surface too.