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 $568.7 billion of the more than $1 trillion in stock index funds as of Jan. 31 -- a 32% increase over last year, but down $39.7 billion from December.

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

Today, we're looking at the ETFs that have best year-to-date performance. We'll then combine that information with the views of the collective intelligence of the 85,000 professional and novice investors at Motley Fool CAPS, to see which funds our participants have rated as the best.

ETF

YTD Return

1-Year Return

CAPS Rating (Out of 5)

UltraShort Semiconductor ProShares (SSG)

40.33%

20.17%

**

UltraShort QQQ ProShares  (AMEX: QID)

39.95%

1.47%

**

UltraShort Technology ProShares  (AMEX: REW)

38.29%

4.04%

**

iShares Silver Trust  (AMEX: SLV)

32.83%

36.43%

****

PowerShares DB Silver  (AMEX: DBS)

31.47%

36.41%

***

UltraShort Russell2000 Growth ProShares (SKK)

27.53%

15.08%

**

iPath DJ AIG Copper TR Sub-Idx ETN (JJC)

26.96%

NA

NR

PowerShares DB Agriculture (DBA)

26.94%

57.22%

****

ELEMENTS Rogers Intl Commodity Metal ETN (RJZ)

26.37%

NA

NR

United States Natural Gas (AMEX: UNG)

26.15%

NA

****

Source: Wall Street Journal. CAPS Ratings courtesy of Motley Fool CAPS. Returns are cumulative total returns as reported by WSJ. NA = not available, NR = not rated.

Tread carefully here, Fools. Although the market offers many ETFs, few have a long history. A few of these on our list don't 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 solid track record over longer time periods.

A rich vein
Although gold grabs all of the headlines and has been reaching new highs lately, we see that so far this year a number of funds focusing on gold's cousin, silver, have been performing even better. Two of the top five specialize in silver, and four on our list deal with various metals commodities.

The rise of the silver surfer, however, could be just another indication of the market's wobble in response to fears of a recession. That same uncertainty probably also accounts for the outperformance of the UltraShort QQQ from ProShares: As the market declines, this ETF seeks to double the fall as a gain. For example, if the "Cubes" -- a.k.a. PowerShares QQQ (Nasdaq: QQQQ) -- falls 10%, the UltraShort QQQ would like to make a 20% gain on that decline. That's similar to the performance the ProShares counterpart UltraShort S&P 500 (AMEX: SDS) seeks out.

With nearly 500 investors rating this ETF, we find that two-thirds of them are expecting it to succeed -- meaning the markets are expected to fall. As top-rated All-Star goldguru succinctly put it a couple of weeks ago, "Recession is a'coming." And pimpie says that Dell's warning of a pullback in consumer spending will ultimately affect other tech giants, a mainstay of the Cubes, and a tech downturn will thus lead to the index's near-term fall.

Though these CAPS investors may be right about an economic slowdown on the horizon, CAPS player SleazyD writes that if you were to buy UltraShort in real life, it would also cost you in terms of high expenses.

Well, it shorts the [NASDAQ] 100. How can you go wrong at this time. There's no way ... I'd buy this [in real life], what with an expense ratio of about 1%, and the cold black heart [it intrinsically] possesses ... but it'll probably be a money maker for a few weeks.

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? Take a peek at the 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.