When ETFs were first introduced, the intent was to provide an investment product that tracked major indexes like the S&P 500 or the Dow Jones Industrial Average.

After the success of the plain-Jane index ETFs, issuers started tackling major sectors like energy, financials and technology. Next they moved to country-specific ETFs. Then they started getting really creative.

Today, there is a seemingly never-ending supply of ideas that ETF issuers turn into real investment products for public consumption. ETFs have filled almost every niche imaginable, from the clearly relevant to the downright obscure. Below are some niche ETFs that you might look further into to diversify your own portfolio.

IQ Merger Arbitrage ETF (NYSE: MNA)

Mergers and acquisitions have historically been the playground for high-flying Wall Street insiders. But with the advent of the IQ Merger Arbitrage ETF, retail investors can get exposure to these multi-billion dollar deals.

MNA invests in companies for which a public announcement of a takeover has been made. For example, if Conglomerate Corp. makes an announcement that it is buying Little Guy, Inc. for $50 per share, this ETF would accumulate shares in Little Guy whenever its trading price drops below $50. Assuming that the deal goes off without a hitch, MNA would benefit from hundreds or thousands of little profits it locked in by taking advantage of the price fluctuations between the merger announcement and the actual $50 payment to Little Guy shareholders.

Nathan Slaughter, editor at The ETF Authority, says of MNA, 'All in all, the fund looks to be an intriguing option for investors wanting to pocket an easy +6% to +8% return in an average year with little downside exposure.' You can read his full analysis here.

IQ Hedge Multi-Strategy Tracker ETF (NYSE: QAI)

Hedge funds engage in investment strategies that are often times lucrative, but generally off-limits to the retail investor. To even get the opportunity to invest in a hedge fund, you need to be an accredited investor, meaning you need a net worth of at least $1 million and an income of at least $200,000 per year for the past 2 years.

QAI brings hedge fund strategies to the average investor by using the same kinds of tactics that hedge funds use to generate outsized returns, like investing in long/short equity, global macro, market neutral, event-driven, fixed income arbitrage, and emerging markets. That's pretty complex stuff for most individual investors, so QAI may be the only chance that many of us have to supplement our portfolios with those types of instruments. QAI has an expense ratio of 0.75%.

Claymore/Beacon Spin-Off ETF (NYSE: CSD)

CSD is the very definition of a niche ETF -- it focuses solely on stocks that are spin-offs from other companies.

The idea of a spin-off is that certain business are more profitable once they separate from their parent companies, where perhaps they had trouble attracting as much management attention and resources as they should have. And according to a study by former Lehman Brothers analyst Chip Dickson, between 2000 and 2005, spin-offs beat the market by an average of 45% during their first two years, while parent companies beat it by an average of 40% over that same period.

CSD usually holds about 40 spin-off stocks at any given time and it has an expense ratio of 0.65%.

Claymore/Sabrient Insider ETF (NYSE: NFO)

History has shown that investors can profit from following insider buying and selling. After all, who knows the goings-on at a company better than the people running it? In fact, we just published an article touting the benefits of tracking insider trading (click here to read The Profit Secrets of Insider Trading).

If you believe in the investment thesis but aren't necessarily interested in sifting through thousands of public filings and analyst earnings estimates, then NFO may be a good choice for you. NFO tracks the Sabrient Insider Sentiment Index, which is made up of 100 stocks that display 'favorable' insider buying trends. note that small caps and mid caps make up around 80% of NFO's holdings, so you will get disproportionate exposure to smaller firms. The ETF has an expense ratio of 0.65%.

PowerShares Listed Private Equity ETF (NYSE: PSP)

Private equity (PE) firms make boatloads of money if they're able to successfully buy and sell companies. Unfortunately, most PE firms are private, and investors are limited to the same 'accredited' standards as the hedge fund investors mentioned above. But a handful of PE firms trade publicly. PSP invests at least 90% of its assets in the stocks that comprise the Red Rocks Listed Private Equity index, giving you a chance to profit during times of increased LBO and M&A activity.

This ETF invests in between 30 and 60 publicly-traded private equity companies. PSP is fairly liquid and has almost $212 million of assets under management. It has an expense ratio of 0.70%.

PowerShares S&P 500 BuyWrite ETF (NYSE: PBP) & PowerShares Nasdaq-100 BuyWrite (Nasdaq: PQBW)

Both PBP and PQBW employ a covered call strategy. PBP sells one-month covered calls on the S&P 500 while PQBW does the same thing on the Nasdaq 100. Both ETFs have demonstrated that their strategy results in smaller downside during bear markets, though also a muted profit potential during bull markets.

A covered call strategy consists of two parts: 1) owning an underlying stock, and 2) selling call options on that same underlying stock. By selling call options, the investor is able to generate additional income (the premiums received from selling options) which can help offset some of the losses incurred during a falling market.

Unlike hedge fund or private equity investing, individuals don't need to meet any special criteria to take advantage of this tactic on their own. Plenty of retail investors write covered calls as part of their investment strategy. But as with the insider sentiment ETF, sometimes investors are more comfortable outsourcing the work to an ETF manager.