It feels like the perfect moment for some auto-promotion. I developed a billing web solution, that is easy to self-host, and I maintain an instance as a SaaS at fossbill.org. There is also a companion documentation website at docs.fossbill.org.
No one knows about it but still, I am proud of it 
It is somehow related to the topic because it mainly targets hackers, or minimalist users. It should fit perfectly to a freelance developer.