Index funds have long been a Foolish way to gain instant, low-cost diversification without worrying about timing the market. Their ease and convenience may explain the growing popularity of exchange-traded funds -- mutual funds that trade like stocks. According to the Investment Company Institute, ETF assets totaled more than $608 billion of the more than $1 trillion in stock index funds as of Dec. 31.

Originally modeled after index funds, ETFs have gradually narrowed to target specialized slices of the market. That's a boon to investors seeking specifically targeted investments, but it also concentrates the risks of specialization, thus tilting a portfolio away from the diversification that makes index investing attractive.

Today, we're looking at the ETFs that have performed the best over the past 30 days. We'll then combine that information with the views of the collective intelligence of the professional and novice investors at Motley Fool CAPS, to see which funds our participants have rated as the best.

ETF

30-Day Return

1-Year Return

CAPS Rating

Market Vectors Steel ETF  (AMEX: SLX)

21.68%

65.60%

*****

iPath DJ AIG Copper TR Sub-Idx ETN

19.38%

NA

NR

Ultra Basic Materials ProShares 

18.02%

37.72%

**

iShares MSCI Brazil Index  (NYSE: EWZ)

17.36%

73.17%

*****

ELEMENTS Rogers Intl Commodity Metal ETN 

17.01%

NA

NR

SPDR S&P Metals & Mining  (AMEX: XME)

16.86%

31.93%

*****

Ultra Oil & Gas ProShares  (AMEX: DIG)

16.06%

43.34%

***

iShares Dow Jones US Oil & Gas Ex Index  (NYSE: IEO)

15.67%

35.93%

*****

United States  Natural Gas  (AMEX: UNG)

15.46%

NA

****

iShares S&P Latin America 40 Index  (NYSE: ILF)

15.33%

45.53%

*****

Source: The Wall Street Journal. CAPS Ratings courtesy of Motley Fool CAPS.

Tread carefully here, Fools: Although the market offers many ETFs, few have a long history. Some don't even have a one-year performance record, let alone a three-year record -- arguably an important milestone -- so only time will tell whether they can build a solid track record over longer stretches of time.

Feeling gaseous
Occupying eight of the top 10 positions (with Latin American economies rounding things out), metals and energy have been among the best performers this past month. Looking to focus specifically on the natural-gas market in the U.S., appropriately titled United States Natural Gas seeks to reflect the daily percentage changes in the price of natural gas via futures contracts.

Natural gas accounts for about 25% of all energy consumption in the country, but weather plays an important component in demand. The price of natural gas, however, has not been as strong as that of oil over the past several years. But recently, gas has started to catch up, as prices have moved from around $5 per million BTUs at the end of last summer to more than $9 currently. That certainly explains why the ETF that's tracking those prices has done so well lately.

The disparity between the price of oil and the price of natural gas attracted top-rated CAPS investors, such as AndyShea and his 97.27 player rating. Andy believed at the time of his pitch that natural gas was due to close the gap.

Natural gas is far too cheap as compared to crude oil. Yes, crude oil is U.S. dollar-denominated, but that does not explain UNG's significant discount as compared to other energy sources, and namely as compared against crude oil. I expect this will realize modest appreciation in the next 3-9 months. Fundamentals are strong.

Even as prices have moved up, a number of CAPS players, including soho48, continue to believe that because there is still too much of a difference in value between natural gas and other fuels, such as oil and coal, U.S. Natural Gas will show significant appreciation in the future. CAPS player johntartar agrees that the time to close the differential is "past due."

Perhaps that's the reason CAPS bears haven't been able to muster any reasons to bet against this natural-gas pure play.

A basket of opinions
Although ETFs have been around since the 1990s, investors should exercise caution with any ETF lacking a long track record. Over on CAPS, let us know whether you think these ETFs will continue to outperform or whether it's time for new ones to top the lists.

Want the short list of stocks that will help your portfolio outperform? Get 30 days of free stock picks with a trial subscription The Motley Fool's investment services.

Fool contributor Rich Duprey does not have a financial position in any of the funds mentioned in this article. You can see his holdings. The Motley Fool has a world-class disclosure policy that has been around the world and back again.