Two California inventors have begun marketing what they call the world’s first "sport utility bicycle." The Xtracycle is a retrofitted frame extender that fits over the back of your bike and allows you to lug as much as 200 pounds of otherwise unwieldy cargo (surfboards, fish tanks, kayaks, six bags of groceries). "With the Xtracycle," their Web site (www.xtracycle.com) crows, "your bike destinations are limited not by the difficulty of bringing the things you’ll need but by your imagination."