Least Sandpipers (Calidris minutilla), Semipalmated Sandpipers (Calidris pusilla), Western Sandpipers (Calidris mauri) and Sanderlings (Calidris alba) all occur in the US and can easily be confused with each other, especially in winter.
Least Sandpipers, however, have greenish-yellow legs while the others have black legs.

Nahant Thicket Wildlife Sanctuary, Nahant, Massachusetts, USA, 20 September 2009