Beekeeper Studio : Open Source cross-platform SQL Editor and Database Manager. Use Beekeeper Studio to query and manage your relational databases, like MySQL, Postgres, ...
Budibase : Budibase is an open-source low-code platform for creating internal apps in minutes. Supports PostgreSQL, MySQL, MSSQL, MongoDB, Rest API, Docker, K8s
Mockoon : Mockoon is the easiest and quickest way to design and run mock APIs. No remote deployment, no account required, free and open-source.
YunoHost : YunoHost is a server operating system aiming to make self-hosting accessible to everyone.
Imagine running your favorite Windows applications and drivers in an open-source environment you can trust. That's ReactOS. Not just an Open but also a Free operating system.