MySQL - The world's most popular open source database
Freeplane - Freeplane is a powerful and free software for building mind maps.
PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
SQLite - SQLite Home Page
SuperNotecard - Introducing SuperNotecard. SuperNotecard is an online writing tool that features virtual notecards to help arrange facts or scenes, track details, organize paragraphs, and clarify your composition process.