Worth: $81 billion
Home: Medina, Washington
Mr. Gates, 60, spends most of his time at his 66,000-square-foot Medina, Washington, mansion, nicknamed Xanadu 2.0 after the title character’s estate in Citizen Kane. The mansion overlooks Lake Washington. It took Mr. Gates seven years and $63.2 million to build this house, which is filled with lots of high-tech features. He purchased the lot for $2 million in 1988, but it’s now worth an estimated $170 million, according to public records.