This is an eastern garter snake (Thamnophis sirtalis), a nonpoisonous species. I ccasionally come across them usually near creeks in Black Hill Regional Park, Germantown, Maryland. I photographed this one in May 2000. It was about a half a meter long (the tulip poplar leaf provides a scale).
Thamnophis sirtalis is a widespread species with many color forms. For more information, try these links.