Historian Ed O'Donnell, author of the book SHIP ABLAZE, has a webpage about the Slocum, and I'll quote from it here:
"Ask any New Yorker to name the city’s greatest disaster before September 11, 2001 and invariably they offer the same answer: the Triangle Shirtwaist factory fire of 1911. That tragic event garnered international headlines as 146 young immigrant women lost their lives in an unsafe garment factory. Yet even though it is certainly Gotham’s most famous disaster, it runs a distant second to a much larger catastrophe which occurred only seven years earlier. On June 15, 1904, more than 1,000 people died when their steamship, the General Slocum, burst into flames while moving up the East River. It was the second-most deadly fire (after the Peshtigo fire of 1871) and most deadly peacetime maritime disaster in American history."
For more information about the tragedy, see O'Donnell's excellent webpage about the tragedy, The General Slocum Disaster, and also the Wikipedia entry on the General Slocum.