Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to invest in the stock market's largest companies, the Rydex Russell 1000 Equal Weight ETF (NYSE: EWRI) 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 ETF tracks the Russell 1000 index, which features about 1,000 large-cap companies, thereby going well beyond the S&P 500's 500-company range. It goes another step further, too. While many indexes portion their holdings according to market capitalization, with the largest companies making up more of the index's value, this ETF is equal-weighted in a particular way. Each industry within it receives equal weighting, and then its components are equally weighted, as well. The end result is that the component companies can have very different weightings, but no industry overshadows others or is underrepresented.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The Rydex ETF's expense ratio -- its annual fee -- is a relatively low 0.4%. The fund is relatively 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.

This ETF doesn't have much of a performance to assess, as it's extremely young. Year-to-date, it's ahead of the S&P 500, but 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.

What's in it?
Plenty of large companies had strong performances over the past year. Green Mountain Coffee Roasters (Nasdaq: GMCR), for example, soared 63%. It keeps frustrating its detractors, who see it as overvalued and worry about a price war. The company has been striking deals with major coffee makers such as Starbucks and Dunkin Donuts to offer their ground beans in the K-cup format.

Advancing 27% was tobacco giant Altria (NYSE: MO), which some have deserted in favor of its global counterpart, Philip Morris International. While the ranks of smokers are shrinking in the U.S. and taxes and regulations are increasing, foreign smokers promise faster, and less fettered, growth. (Both companies sport attractive dividends.)

Level 3 Communications (Nasdaq: LVLT) gained 19%, investing heavily in its future as it aims to boost its fiber-optic network's capacity to meet growing demand. Bears are still uncomfortable with the company's debt load, though.

Other companies didn't do as well last year but could see their fortunes change in the coming years. Consider Dendreon (Nasdaq: DNDN), for instance, down 56%. It's been a roller-coaster ride for shareholders in recent years, as the company's prostate-cancer drug Provenge earned FDA approval, debuted on the market, put many off with its hefty price tag, saw lackluster demand, and at one point wasn't able to meet demand. But sales are growing now, and Dendreon has a new CEO, so its future may be brighter.

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.