Low fees have become one of the most powerful advantages in modern investing. Some of the largest ETFs tracking the U.S. market now charge as little as 0.03% annually, giving investors extremely efficient exposure to hundreds or even thousands of companies. In this article, we take a closer look at four giant funds that combine enormous size, strong liquidity and minimal costs. For long-term investors, these ETFs often represent the simplest gateway to the entire American equity market.

At first glance, an expense ratio of 0.03% per year vs. 0.20% per year may seem like a negligible difference. A closer look at the numbers, however, reveals just how significant an impact the level of fees can have on a portfolio's total return over a 20- or 30-year horizon. That's why low-cost index ETFs from giants like Vanguard, BlackRock $BLK and State Street $STT have been one of the fastest growing investment products in the world since the early 2000s.

The entire global ETF industry attracted a…