I sell productivity tools - Chrome Extensions and a SaaS - that solve my own problems in a niche. All of them run subscriptions with varying kinds of free/trial access, depending on the audience and the cost to me.

If you're curious on the details, I wrote a post recently about getting to $100/month with them: https://www.simonmweber.com/2017/01/09/side-project-income-2....