I maintain the service virtualization tool mountebank, which competes with commercial tools in the same space (IBM, CA, Parasoft, SmartBear), but with a better developer experience and a lot less in licensing costs.

Mountebank is free to use, but takes a significant amount of time to support and maintain. In addition to developing new features, I regularly respond to public support questions. I've done this for the past seven years, despite increasing time commitments from my day job and family.

The commercial tools can easily run into the hundreds of thousands of dollars per annum. Your contributions make ongoing maintenance possible, and, compared to the alternative, is a bargain.