Investing in the S&P 500 (SNPINDEX: ^GSPC) is popular, especially through exchange-traded funds (ETFs) like the Vanguard S&P 500 ETF (NYSEMKT: VOO). The S&P 500 index is made up of the 500 biggest companies in America. Invest in the S&P 500, and you become a part owner of 500 large businesses, instantly diversifying your portfolio. Here are the top 10 holdings of the Vanguard S&P 500 ETF and their share of the index:Stock Weighting 1. It's also an S&P 500 index fund, but one that weights each of the 500 companies equally and rebalances quarterly.