AppGyver

AppGyver (now part of SAP) is a free, professional-grade low-code platform for building web and mobile apps. Its Composer Pro tool uses a visual logic editor and supports custom code components. It's aimed at professional developers and enterprise teams building complex applications. It is completely free to use.

It scores for its powerful, pro-level features and being truly free, though it has a steeper learning curve than consumer-focused no-code tools, targeting a more technical audience.