Software radical Richard Stallman helped build the Linux revolution. Now he threatens to tear it apart. The free Linux operating system set off one of the biggest revolutions in the history of computing when it leapt from the fingertips of a Finnish college kid named Linus Torvalds 15 years ago. Linux now drives $15 billion in annual sales of hardware, software and services, and this wondrous bit of code has been tweaked by thousands of independent programmers to run the world's most powerful supercomputers, the latest cell phones and TiVo video recorders and other gadgets. But while Torvalds has been...