The iShares S&P 500 ETF (quote: IVV) has an expense ratio of 0.04%, is offered commission-free on many brokerages, and based on some quick calculations, follows the official index to within about 0.5% returns. You're right that it's not completely "free", but the total "cost" is only about half a percentage point, meaning you can pretty closely follow the index in the long run with a single ETF purchase. I don't know what could be easier. :)