
Giant clam shells make great
Seashell Bowls and
Seashell Planters, and look stunning by themselves too. It's a good thing that most of the giant clam shells used for decor are resin or stone cast, because they are being over harvested as is for food, decor and the aquarium trade. Here's a list
Where to Buy Faux giant Clam Shells.

How creative! This resin clam shell is anchored in a
Bathroom over the tub and planted with Dendrobium orchids.

In
Ali Wentworth's Home in Washington DC, you find one in the fireplace.

Filling the space underneath a side table. Via
Martha Stewart.

A seashell planter helps soften a bedroom done by
Phoebe Howard.

Designer
Barry Dixon placed a giant seashell in an enclosed shower.
Tobi Fairley created a center piece with an elegant white Orchid. See
here.

And to answer Erin's question (see comments), you can have a shell sink custom made at
Jupiter Granite.

And here's an example of a sink via
House Beautiful.

And here a few interesting facts about
Real Live Giant Clam Shells via National Geographic:
The giant clam gets only one chance to find a nice home. Once it fastens itself to a spot on a reef, there it sits for the rest of its life.
These bottom-dwelling behemoths are the largest mollusks on Earth, capable of reaching 4 feet (1.2 meters) in length and weighing more than 500 pounds (227 kg). They live in the warm waters of the South Pacific and Indian oceans.
Giant clams achieve their enormous proportions by consuming the sugars and proteins produced by the billions of algae that live in their tissues. In exchange, they offer the algae a safe home and regular access to sunlight for photosynthesis, basking by day below the water's surface with their fluted shells open and their multi-colored mantles exposed.
Giant clams have a wildly undeserved reputation as man-eaters, with South Pacific legends describing clams that lie in wait to trap unsuspecting swimmers or swallow them whole. No account of a human death by giant clam has ever been substantiated.