Common Tern, Sterna hirundo
6/5/06, Sandy Point, West Haven, CT
Common Terns are indeed common as nesters along the Atlantic coast; they are relative rarities, mostly in fall, near the California coast where I live.
Common Tern, 6/5/06, Sandy Point, West Haven, CT
Common Tern, with fish, 5/14/11, Regent's Park, London
The Common Tern shown perched having its breakfast above and below was photographed across the pond in Regent's Park, London.
Common Tern, 5/14/11, Regent's Park, London
Common Tern, flying, 7/15/10, Pine Point, Scarborough, ME
The birds shown above and below here were photographed on the southern Maine coast in July.
Common Tern, flying, 7/15/10, Pine Point, Scarborough, ME
Common Tern, non-breeding plumage, 9/26/08, Jetty Road, Moss Landing, Monterey Co
A few Common Terns like this one are seen every fall during migration on or near the California coast.
 
Common Tern, non-breeding, flying, 9/27/08, Monterey Bay, Monterey Co
This fall Common Tern was seen on a pelagic trip on Monterey Bay in fall, where either a Common or an Arctic might be seen. The dark wedge on the primaries identifies this as a Common.
Common or Arctic Tern, juvenile, 9/22/06, Salinas State Beach, Monterey Co
I posted this picture on a birding listserv for comment, inquiring if it showed a Common Tern, and an expert noted that the short bill suggested a juvenile Arctic, especially if the apparent dark leading edge to the wing was an artifact of the photo angle. An Arctic would be very rare, a Common not nearly so rare on the central California shore, so I have it down as a "Commic" Tern (Arctic and Common are often hard to separate when seen in flight), and have posted it both here and on my Arctic Tern page.