Tax Lien certificates. Basically, you're giving an extension to someone who is delinquent on their property taxes, and ensuring that the local government, who probably very much needs predictable funds, collects them in a timely manner.
Some of these are cheap, in the hundred dollar range, which makes it easier to get started even if you don't have a lot of money to invest. Terms and availability depend on the area you buy them from. Interest rates can be very high, around 20% in some areas. In some cases (likely foreclosures), you can have a good chance of becoming the owner (or part owner) of the property, which can be massively profitable (but also a hassle).
On the other hand, some property is not that valuable, so you need to do some research. The lack of secondary markets for these makes them rather hard to sell early. And if you don't live in an area that offers good terms, you may have to travel to find the good deals, which is an expense. Some counties do offer auctions online, but you'd still need to do some research on the property.