Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you want to add some proven and profitable giants to your portfolio, perhaps to balance some small caps, the Vanguard Mega Cap 300 Index ETF (NYSE: MGC) could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The Mega Cap ETF's expense ratio -- its annual fee -- is an ultra low 0.12%. (Vanguard is known for its low fees.)

This ETF hasn't performed spectacularly so far, but it's also very young, with just a few years on the books. It underperformed the S&P 500 by a smidge over the past three years and outperformed it by a smidge over the past year. As with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

With a low turnover rate of 8%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Plenty of huge companies had strong performances over the past year. Apple (Nasdaq: AAPL) surged 64%, selling 35 million iPhones and nearly 12 million iPads in the last quarter alone. The innovative company has invented whole new product categories and then gone on to dominate them. Some fear that it will eventually stumble, but the stock actually still looks rather inexpensive.

Philip Morris International (NYSE: PM) gained 32%, and bulls like that fact that while smoking is waning in the U.S., it's growing in other parts of the world, where the population is also growing briskly. The company is a solid dividend payer, recently yielding 3.6%, and it's buying smokeless-nicotine technology to help it develop new product offerings.

Other companies didn't do as well last year but could see their fortunes change in the coming years. Cisco Systems (Nasdaq: CSCO) shed 1%, for instance. The company has been pulling itself together, eliminating less profitable businesses and focusing on core strengths. With solid revenue growth and economies of scale to exploit, the company is positioned well. A recent stock-price tumble only makes it a more intriguing opportunity.

Finally, General Electric (NYSE: GE) dropped 2%. It offers very broad diversification, as well as exposure to new and developing technologies, such as alternative energy. It also offers a strong balance sheet and a growing dividend (recently yielding 3.6%). The company is expanding internationally and stands to benefit as global infrastructure projects get under way.

The big picture
A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.

Apple isn't the only way to profit off the explosive mobile market. Check out our special free report, "The Next Trillion-Dollar Revolution," that details a component play inside mobile phones that also is a market leader in the exploding Chinese market.