This past Saturday, I saw my first Western Sandpiper (Life Bird #293). The juvenile bird was foraging on a small beach in Biddeford, ME. Western Sandpipers are both New England's largest (though not by much) and least common peep. (As you may have guessed, Western Sandpipers are common migrants on the west coast of the United States.) I've observed probably thousands of Semipalmated Sandpipers and hundreds of Least Sandpipers in the last few years, but until this weekend I hadn't identified a single Western.
To learn more about Western Sandpipers and how to recognize them in the field, I recommend The Shorebird Guide (2006) by Michael O'Brien, Richard Crossley, and Kevin Karlson, or another trusted bird guide. For a brief online write-up, check out All About Birds, which includes some audio samples.