On this Labor Day there's no denying that the labor movement is not what it used to be. But there are groups that are filling the void left by declining union membership. Some 225 "worker centers" have sprung up to focus on America's 22 million immigrant workers, both legal and undocumented. The growing immigrant work force is especially vulnerable to abuses, including a lack of health insurance, workers compensation and sometimes even a lack of payment for work performed. In Texas, the Workers Defense Project has taken on some powerful corporate players — including one of the biggest, Apple computers. Steve Greenhouse reports for the New York Times.