The Digital Millennium Copyright Act. Its "safe harbor" provision is what draws the line between pirates and legally legitimate Web companies. That line isn't always crystal clear, as the ongoing saga of Grooveshark demonstrates. Few would call the DMCA perfect, but its attempt at modestly redefining copyright for the digital age has had a major impact on the way the Web works, and in many cases has enabled innovation to flourish. Without it, sites like YouTube might not be what they are today.