In opening up niche-investment opportunities, exchange-traded funds have taken the investing world by storm. But with more than 1,000 ETFs out there -- and new ones opening all the time -- it's almost impossible even to keep track of all your choices, let alone figure out which ETFs are the best.

As the ETF world gets more crowded, ETFs that focus on certain niches have proven to have the best chances of finding success. Below, let's take a look at four exchange-traded products that have gotten substantial attention from investors, with an eye toward figuring out whether they belong in your portfolio.

PowerShares S&P 500 Low Volatility ETF (NYSEMKT:SPLV)
As the market has skyrocketed to record highs, many investors have gotten nervous about whether stocks can continue their bull-market run. To protect themselves, they're looking for stocks that are prone to fall less dramatically if the market reverses downward.

The PowerShares ETF focuses on low-volatility stocks, which tend to have price movements that are less severe than the overall market. That can leave you underperforming the market during bull markets, but the hope is that when stocks fall, low-volatility stocks will fall less. In addition to that protection, the stocks that tend to have low volatility also often have attractive dividend yields, giving investors a double benefit from the ETF.

For investors who really need as much income from their portfolios as they can find, mortgage REITs have been an obvious answer to low interest rates. By using those low rates to their advantage by boosting their leverage levels and buying mortgage-backed securities, mortgage REITs have produced not only amazingly high yields but also capital gains in some cases.

The iShares ETF isn't composed solely of mortgage REITs, also including some real-estate management companies and other niche real-estate investments. But that diversification may actually help the iShares ETF compared to more focused mortgage-REIT investments like Market Vector Mortgage REIT ETF (NYSEMKT:MORT) if the long-awaited rise in interest rates eventually comes to pass. Until then, though, ETF shareholders are likely to keep reaping double-digit dividend yields.

Aberdeen Asia-Pacific Income Fund (NYSEMKT:FAX)
Low interest rates have made investing in bonds very difficult for investors. Buying a 10-year Treasury right now involves locking in a rate below 2% through 2023, leaving yourself exposed to the growing potential for rising rates in future years.

But interest rates in other countries are higher -- in some cases, such as Australia, much higher -- and that has helped raise awareness of foreign bonds denominated in other currencies. The Aberdeen fund is a closed-end fund, which trades in much the same way as an ETF. But even though a majority of its holdings are in sovereign debt of Australia and other Asia-Pacific countries, it currently pays a dividend yield of 5.5%. Right now, it's even trading at a rare discount to its net asset value, despite the fact that it has routinely fetched a modest premium to NAV throughout the past nine months. If you're willing to take on currency risk, then foreign bond exposure can be a ticket to higher income.

iShares S&P US Preferred Stock (NASDAQ:PFF)
Preferred stock is largely ignored among mainstream investors. But the higher income yields that preferred stock usually offers and its greater stability make it an attractive alternative for those seeking more income from their portfolios.

One concern about the iShares ETF is that it has a disproportionately high concentration to financial companies. Yet as banks and other financial institutions have seen their prospects rise in recent years, concerns about preferred-stock quality have subsided, making the ETF and its 6% yield look more promising to income investors.

Investing for your success
ETFs are a great tool to help you get broad-based exposure to major asset classes, but niche ETFs can also help you spice up your portfolio by allowing you to make targeted plays on certain investing themes. The four ETFs discussed above aren't guaranteed to rise in value, but they will give you the best chance to profit if the respective investing rationales behind them turn out to be true.

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.