S&P 500

Standard and Poor's 500 Index is a capitalization-weighted index of 500 stocks. The index is designed to measure performance of the broad domestic economy through changes in the aggregate market value of 500 stocks representing all major industries. The index was developed with a base level of 10 for the 1941-43 base period.

Standard & Poor started around 1957, and includes 500 listed companies in the US from large to small, and is a good representation of 98% of the businesses.  S&P500 is a good indication of the US economy.