Russia has made a comeback in 2015. Image source: Wikimedia Commons.

Exchange-traded funds have made it easier than ever to invest in the broad market or in particular niches within it. Yet many ETFs have taken big hits in 2015, as the stock market has become more turbulent and sent many key sectors into tailspins, especially areas like energy and materials. Still, a few ETFs have produced impressive gains so far this year, and some of the areas that they target might well surprise you. Let's take a look at four popular ETFs that have produced double-digit percentage increases even with the overall market down so far for 2015.

Biotech keeps booming
The biotech industry has produced amazing performance in recent years, and it has kept up its record of outperformance so far in 2015. The SPDR S&P Biotech ETF (NYSEMKT:XBI) is up about 29% for the year, and the iShares Nasdaq Biotechnology ETF (NASDAQ:IBB) has also done well, with gains of almost 20%.

Source: FDA.

The reason for the disparate performance has to do with the relative weightings of biotech stocks in each ETF. The SPDR ETF follows an equal-weight strategy, which means it benefits more when smaller stocks in the fund soar as a result of positive clinical trial news or a takeover bid. By contrast, the iShares ETF holds larger proportions of its assets in major biotech stocks, most of which have done reasonably well in 2015 but not nearly as well as high-flying smaller companies. With choices for aggressive or conservative investors, biotech has plenty of opportunities for those willing to pursue them.

From Russia with love
The strong dollar has made it tough for overseas markets to compete. Yet some of the best returns in international markets come from an unexpected source -- Russia -- with the Market Vectors Russia ETF (NYSEMKT:RSX) up 14% so far in 2015.

You'd think that Russia would be one of the last markets to rebound under current conditions, given the country's reliance on natural resources and the poor conditions in key commodity markets like oil and base metals. Yet it's important to remember that the Russia ETF lost half its value in 2014, and so this year's outperformance has only put a very small dent in the losses that long-term investors continue to suffer. As long as oil prices remain low and other key commodity markets fail to rebound, it could be a long time before this Russia ETF and other investments in the region get back to where they traded just a year or two ago.

Housing keeps recovering
The housing market's collapse was a major cause of the financial crisis, but housing has recovered sharply in recent years. The iShares US Home Construction ETF (NYSEMKT:ITB) is up 11% so far in 2015, defying concerns that a coming rate hike could cause the sector to slow.

The strength of this housing ETF comes only partly from homebuilder stocks, which are well represented in the fund. Yet you'll also find several suppliers of building materials for home construction within the ETF, and most of those companies have seen double-digit percentage moves upward in their share prices as well. Some are growing increasingly concerned that the housing market might have come too far too fast, yet most agree that the conditions aren't nearly as extreme as they were in the run-up to the housing bubble in the mid-2000s, and they hope that we'll avoid a similar outcome this time around.

Challenging markets can make it tough to find winning ETF investments, but there will always be ETFs that manage to move higher even when major markets are in decline. These four ETFs won't necessarily outperform their peers forever, but they do show that ETFs that are in the right place at the right time can post impressive returns no matter what the market is doing.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.