Investing for income is a popular strategy, and for good reason. Who doesn't love the idea of passive income paying your living expenses? But pulling it off can be more complicated because you can't trust every dividend stock you find.

That's where exchange-traded funds (ETFs) can help. These are buckets of individual stocks that trade under a single ticker symbol. That means instant and easy diversification for your portfolio, which helps you sleep better at night.

But which ETFs are best for income-focused investors?

With $723, you can buy and hold one share of each of the following ETFs. 

1. The ultimate ETF for following the broader market

The SPDR S&P 500 ETF (SPY 0.95%) was the first ETF fund traded on the U.S. markets. Today, it's one of the most popular ETFs in existence. The premise of this fund is simple. It tracks the S&P 500, an index of approximately 500 of America's most prominent and brightest corporations.

Following the S&P 500, it has historically paid very well. Since its creation in the early 1990s, the ETF has turned a $10,000 investment into over $211,000. Additionally, the S&P 500 adds and removes companies, so investors know the index constantly holds Wall Street's most prominent businesses.

It's not the highest-yielding ETF. Distributions yield roughly 1.3% today. However, without dividends, that $10,000 investment would have grown to just $118,000 over the same time frame, so don't underestimate it.

The ETF currently trades for $522.

2. A high-yielding ETF for conservative investors

The iShares 20+ Year Treasury Bond ETF (TLT 0.52%) is an excellent option for those looking to maximize their income. The fund's distributions currently yield 4.4%. Instead of stocks, the fund tracks an index of U.S. Treasury bonds. Specifically, the fund focuses on long-duration Treasury bonds of at least 20 years.

A Treasury bond is like the U.S. government's "I owe you" note. It's how the government borrows money. Investors get a return on that bond as an incentive to buy it. That's called the coupon rate. The iShares 20+ Year Treasury Bond ETF is an easy way for investors to add Treasuries to their portfolio without picking specific bonds and monitoring their maturities.

As long as America always pays its bills, and it always has to date, the fund is an excellent choice for those looking to diversify their portfolio away from owning just stocks.

This ETF currently trades for just under $94.

3. Corporate debt for high yields

The iShares iBoxx $ Investment Grade Corporate Bond ETF (LQD 0.38%) is another way investors can earn passive income from others' debts. It's similar to the previous fund, except it focuses on the private sector. The fund invests in corporate bonds and yields an impressive 5.3% on its distributions.

The most significant risk in any bond or bond fund is that the borrowers don't pay back their debts, defaulting on the bonds. It's a genuine risk, but this fund helps alleviate that concern by focusing on bonds with investment-grade ratings. In other words, the companies issuing these bonds have good credit.

Again, this fund is not for people looking to maximize their capital returns. Over the past decade, it has returned an annualized average of 3%. But it's another option for investors diversifying and looking for income from their investment portfolio.

This ETF currently trades for just under $109.