All three of these picks come from index specialist Vanguard. Image source: Vanguard Group.

Index funds are a great low-cost way to invest in the stock market. Whether you go with broad-based index funds that cover the entire market or specialized funds that focus on a particular piece, investing with index mutual funds and exchange-traded funds can give you the exposure you want at a price you'll like. To give you some ideas on where to look, we had three of our Motley Fool contributors present their ideas for index funds they like right now. Read on and see if their picks inspire interest for you.

Andres Cardenal: Vanguard Emerging Markets Stock Index Fund Admiral Shares (VEMAX 0.09%) is a low-cost index fund offering diversified exposure to emerging-market stocks. Many investors tend to stay away from emerging markets because of their higher volatility, however, it's important to consider that these regions of the world have a lot to offer in terms of diversification and potential returns.

Many emerging countries are going through considerable economic turmoil lately. The economy in China is decelerating, Brazil is in the midst of a deep political and economic crisis, and falling oil prices are hurting Russia and other energy exporters. However, chances are that emerging markets as a diverse group of nations will continue on the path to increased prosperity over the years ahead. For this reason, short-term uncertainty in emerging markets can be a source of opportunity for long-term investors.

While emerging market stocks are in fact more volatile than companies in developed markets, investment decisions need to be made with the whole portfolio in mind. If you're heavily concentrated in the U.S., adding some emerging-market stocks to the mix could be a smart way to reap the benefits of diversification.

Vanguard Emerging Markets Stock Index Fund Admiral Shares offers exposure to almost 4,000 companies across a wide a spectrum of emerging markets, including countries such as China (27.3% of the portfolio), Taiwan (14.5%), India (12%), South Africa (8.6%), Brazil (8.3%), Mexico (5.2%), and Russia (4.5%), among several others. Importantly, the fund has a remarkably low annual expense ratio of 0.15%, 90% lower than the average expense ratio of funds with similar holdings.

Jason Hall: Over the past few years, the strong overall performance of the stock market has been better than the Vanguard Dividend Appreciation ETF (VIG 0.29%). However, since its inception, this fund has been a strong performer:

VIG Total Return Price Chart

VIG Total Return Price data by YCharts

And while that's no guarantee that this fund will outperform the market in years ahead, there's a lot of evidence that the kinds of companies owned in the fund, which tracks the NASDAQ US Dividend Achievers Select Index, should lead to strong long-term returns. The companies in this fund are primarily large, well-capitalized companies that pay steady dividends and have a track record of regular annual dividend increases. Over time, the compounding growth of those dividends can really boost your returns. 

One could argue that the recent jobs report doesn't bode well for many of the companies in this fund, of which more than 40% are consumer goods and services businesses, but many of those companies, including Johnson & JohnsonCoca-Cola, PepsiCo, and CVS Health, a major portion of their businesses are largely recession-resistant and will sustain any short-term downturns just fine. 

If you're looking for a great long-term holding, now's a good time to buy Vanguard Dividend Appreciation ETF and then let years of compounding dividend growth add up. 

Dan Caplinger: The index funds that Andres and Jason have set forth are good for all seasons, but I wanted to focus on an exchange-traded fund that is more timely. The Vanguard Energy ETF (VDE 1.20%) is an exchange-traded fund that focuses on energy stocks, and most investors know quite well just how hard-hit the energy sector has been over the past year and a half. With crude oil prices having plunged and natural gas prices remaining low as well, it's been a hard market to navigate for oil and gas producers and the energy services companies that serve them.

Recently, though, energy has shown signs of having bottomed out. Since falling into the $20s briefly at the beginning of the year, crude oil has risen back toward the $50 per barrel level, and the Vanguard Energy ETF has risen by about a third from its lows of the year. Yet even with those gains, the ETF is still down by nearly a third from its highs in 2014. Crude oil prices wouldn't have to regain all their lost ground in order for the Vanguard Energy ETF to keep producing share-price gains. Value investors might begrudge the fact that they missed the absolute bottom, but with new signs of life, now is a good time to consider adding this sector ETF to your index fund portfolio.