While dining with friends with whom he resided, the pilot told them that he was 'going flying'. At the completion of the flight he would fly over the house before he returned to Palo Alto Airport (PAO). The pilot twice circled the residence which is about 7 miles southwest of PAO at a low altitude and then proceeded west. Ground witnesses reported that when the airplane was about 1/2 mile west of the residence it entered a 70 degree nose high attitude. The maneuver resembled a left hammerhead stall except before it reached the nose down attitude it entered a left spin. One witness close to the accident site reported that the airplane had stopped its spin rotation and was in about a 45 degree nose down attitude at impact. The witnesses near the accident site reported that one of the engines sounded irregular. The post crash investigation disclosed no evidence of any airplane or engine preexisting malfunctions or failures.