MySQL - The world's most popular open source database
Xmind - Xmind is a brainstorming and mind mapping application.
PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
MindMeister - Create, share and collaboratively work on mind maps with MindMeister, the leading online mind mapping software. Includes apps for iPhone, iPad and Android.
SQLite - SQLite Home Page
Coggle - Coggle is a simple, beautiful, powerful way of structuring information.