There are three common stock indices that allow investors to invest in the overall stock market at low costs: SPY (tracks the S&P 500), DIA (tracks the Dow 30), and QQQ (tracks the Nasdaq 100). These indices, called Spiders, Diamonds, and WEBS, can be traded like individual stocks. The document also lists sector-specific and international indices that track segments of the market, such as energy, financials, technology, Japan, and the UK. Students can use websites like Yahoo Finance to view charts comparing the performance of these indices to the overall stock market in recent periods.