Great Investment Resource
The folks at Callan Associates have constructed a truly unique look at investment returns over the last 20 years. Visit this link (you'll need Adobe Reader) and look at how the returns from various investment indices change from year to year. Conclusion: owning the S&P 500 index is not enough diversification for most portfolios.
I have purchased more focused index funds that cover sections of the market like: small cap value, large cap growth, etc. Also, don't forget international and real asset exposure.
Check out the annual endowment report for Yale... these guys really know what they're doing.