When it comes to digital products, e-books and apps come to mind first. Once you make it, it can be a model that you can sell again and again and make a good income over time.
You can also make a digital product this way, for example, if you are a designer. You can prepare an icon set for iOS. You can make a single one at first and sell it. Afterwards, you can also establish a digital brand with more than one set.