Word of mouth. If you make happy customers, they'll readily tell others.

But the truth is most modern products aren't good enough to earn word of mouth.

A good example of how to work it right is Steam: while it is not perfect, most discussions give them benefit of doubt because most of the time they do work for the best interest of their customers, not just themselves.

Eeyup. Costco does zero advertising, and yet everyone knows about Costco. Why? Because they're good. In reality, the prices don't always work out, but they have so many other nice things: opticians, tires, a food court (with loss leaders!), rotisserie chicken (also a loss leader), solid products, etc. Costco exists to make money, sure, but it doesn't feel like they're trying to screw you. I can't say that about 99.9% of companies now.