Many were counting WiMAX out, but it appears that it has one last chance with the Sprint experiment. Through Clearwire, an affiliated company in which Sprint owns a 51 percent stake, Sprint is now offering the faster data service on laptops in Baltimore, Portland, Ore., and other cities for a total population of eight million people. By the end of the year, the service will be in 25 markets, including Chicago, Philadelphia and Dallas. A year after that, it hopes to reach about a third of the country’s population, including New York and San Francisco.