Read our handy guide to the stocks in the S&P 500 to see which of these are the biggest by weight. The S&P 500 is an index of brands with a massive total market cap, and it features all of the largest companies in the US. Remember that CAPEX gives you the chance to invest in CFDs for many of these S&P 500 stocks so that you can get a return from your investment without having to actually hold the underlying asset.