Bill Gates' 66,000-square-foot compound is built into a hillside on the edge of Lake Washington, near Seattle. Its enviable amenities include: a 60-foot swimming pool with an underwater music system, a 2,500-square-foot gym and a 1,000-square-foot dining room, which seats 24. For a personal touch, out-of-shape visitors can skip the 84-step hike to the ground floor and opt for an elevator ride instead.