GnuCash

GnuCash is a free, open-source, professional-grade double-entry accounting software for desktop (Windows, macOS, Linux). It is incredibly powerful, handling invoicing, banking, stocks, income/expense tracking, and detailed financial reports. As open-source software, it is completely free to use forever with no restrictions. However, it is a desktop application with a steeper learning curve and lacks automatic bank feeds and seamless cloud sync (though file-based sync is possible).

It's best for tech-savvy, financially literate solopreneurs who prioritize control, data privacy, and a one-time $0 cost over cloud convenience and automation.