Lesser Maple Spanworm, Speranza pustularia. A few of these caterpillars, along with the leaves they were on, were blown out of our front yard's Silver Maple tree during yesterday evening's thunderstorm. They are members of the Geometridae, the inchworm family.
The eggs overwinter on the branches of a tree, typically a maple. In May the caterpillars hatch and begin feeding on leaves. The adult moths can be found throughout the summer.
It's not a very distinctive caterpillar. The most distinctive identifying features are the light colored stripes running along the upper sides of the caterpillar's body (the sub dorsal stripe).