If you just want to track to S&P 500 index, Fidelity Spartan 500 Index Investor is the low-cost choice for you.

The fund's struggles in 2005 don't dampen its long-term appeal. In that year, many active fund managers added strong-performing foreign stocks and smaller-cap domestic issues, which outperformed this fund's domestic mega- and large-cap-dominated portfolio. In addition, many of its large-blend rivals held Google GOOG at or near the top of their portfolios, capturing some part of the stock's stellar 115% gain last year. (Google was only added to the index in April 2006.)

But investing in index funds requires a long-term horizon, and despite a poor recent showing versus the category, the case for this fund remains strong. Over the past 10 years the fund has topped 70% of its category peers. And Fidelity's move to cap the fund's fees at an ultralow 0.10% (0.07% for those with $100,000 to invest in the Advantage share class) in 2005 will give it a further head start going forward. Moreover, the fund's low-turnover profile makes it a tax-efficient way to gain large-cap exposure while keeping down trading costs--which erode returns but aren't included in expense ratios.