S&P 500: Total and Inflation-Adjusted Historical Returns. Historical prices for the Standard & Poor's 500 stock-market index can be obtained from websites like Yahoo Finance, using the ^GSPC ticker, or Google Finance, with .INX.
Yahoo can even graph the series since 1950. Those numbers, and their corresponding graphs, are useful for evaluating the past performance of stock investments, because the S&P500 index is well regarded as a proxy for the large-cap stock market. Nevertheless, to study the real profitability of the market, we need to average and graph not only the price, but the effect of dividend distributions and inflation as well.
Dow Jones Industrial Average (1900 - Present Monthly) - Charting Tools. Dow Jones Industrial Average Index ($INDU) - Stock chart, Index chart. StockCharts.com - Simply the Web's Best Financial Charts.