X-treme Customization is everywhere. Not long ago cellphone users were satisfied with proclaiming their questionable uniqueness by downloading musical ringtones from their favorite pop artists. Now, "it is not enough to choose a ringtone," says Harmony Line CEO Richard Resnick. A person has got to make his own. Before the Industrial Revolution, everything was custom-built and expensive. In the 1980s plummeting technology costs and the Internet gave people "mass customization," spawning made-to-order jeans and pick-your-parts PCs. For every customization skeptic, there are ten evangelists such as Ben Serotta, whose Saratoga Springs, NY, company, Serotta Competition Bicycles, makes customized bikes.