The Vanguard S&P 500 ETF ( NYSEARCA:VOO ) is one of the most popular exchange-traded funds, offering investors broad exposure to the 500 largest U.S. companies. Tracking the S&P 500 Index, VOO is a low-cost, market-cap-weighted fund that reflects the performance of America's economic heavyweights. With an expense ratio of just 0.03%, it's a favorite for long-term investors seeking diversified growth.