Invoice Ninja

Invoice Ninja is a wildly popular, highly disruptive, and fiercely developer-friendly platform that absolutely dominates the "Open Source and Self-Hosted Invoicing" sector. While companies like Zoho control your data on their cloud servers, Invoice Ninja built a massive cult following by allowing tech-savvy freelancers and agencies to download the software and run it entirely on their own private servers for ultimate control. The absolute core superpower of Invoice Ninja is its "Self-Hosted Privacy and Developer Customization." For a web development agency or a highly privacy-conscious consulting firm, handing all their financial client data to a third-party SaaS company is unacceptable. Invoice Ninja allows them to host their billing system internally, completely white-label the software so it looks like their own proprietary app, and hook into its massive open API. Furthermore, its hosted SaaS version is incredibly affordable. Because it offers staggering customization (HTML invoice design), massive payment gateway support (over 40+ gateways), and true open-source freedom, it is the absolute darling of web developers, IT consultants, and tech agencies.