Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some consistently growing companies to your stock portfolio (perhaps to help you sleep better at night), the Russell Consistent Growth ETF (NYSE: CONG) 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 Russell ETF's expense ratio -- its annual fee -- is a relatively low 0.37%.

This ETF is too young to have a sufficient track record to assess. 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. The fund is small, too, so if you're thinking of buying, beware of occasionally large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

What's in it?
Plenty of consistent growers had strong performances over the past year. One of the most impressive is Apple (Nasdaq: AAPL), almost left for dead not so many years ago. Essentially, by inventing entire new markets (iPods, iPads, etc.), the company has been growing briskly -- and yet still seems undervalued to many. Signaling its confidence in steady cash generation, the company recently initiated a dividend -- but even factoring that in, the company will be left with many billions of dollars with which it might further fuel its growth, via smart investments.

United Parcel Service (NYSE: UPS) has a longer track record for consistency, enjoying economies of scale due to its vast network. Its growth should get a kick from its international investments and also from the spread of online retailing, as it delivers more goods to more people.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Construction equipment giant Caterpillar (NYSE: CAT), down 2%, is now a major mining equipment maker, due to its acquisition of Bucyrus. It's also investing significantly in emerging markets such as China and Russia; it will profit as the global economy eventually heats up again, and is seen as rather undervalued by many.

Energy giant Schlumberger (NYSE: SLB), meanwhile, is far less of a market darling than Apple, but it's among the 10 companies expected to grow most briskly in 2013. Demand for oil and gas isn't likely to evaporate any time soon, and Schlumberger is in the business of helping companies find and extract them.

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.

Learn about the 5 ETFs That Could Soar in 2012. And if you're looking for some great investments beyond ETFs, consider these 12 Dividend Stocks for 2012.