A new, non-partisan, non-profit newsroom producing journalism in the public interest will launch here in January under the name ProPublica. Paul E. Steiger, former managing editor of The Wall Street Journal, will serve as president and editor in chief. ProPublica, when fully staffed in 2008, will include 24 fulltime reporters and editors, the largest staff in American journalism devoted solely to investigative reporting.  ProPublica will be supported entirely by philanthropy and will provide the articles it produces, free of charge, both through its own Web site and to leading news organizations selected with an eye toward maximizing the impact of each article.