MySQL is a popular post-relational database used in many open source projects. It is easy to install and administer, but also fast and efficient. For running a world bank, it may not have the resilience and support of eg Oracle, but in educational contexts it is certainly powerful enough.
MySQL is dual licensed, that is available under either an open source GPL or a proprietary licence. Commercial support, training and several books are avaliable.
Do you use MySQL? Write about your experiences here.