The S&P500 index funds are linked to the S&P500 where there are multiple ways to trade and are arbitraged to the nearest cent. People have mentioned VOO & SPY already but most liquid are futures which are super liquid. Eg yesterday there was 2,394501,116,312=$78 Billion traded in the Chicago e-mini contract alone. Which is more than $3mil a second.

http://www.cmegroup.com/trading/equity-index/us-index/e-mini...