A collection of fun databases for programming exploration

Esther Schindler writes: When you learn a new tool/technology, you need to create a sample application, which cannot use real in-house data. Why not use something fun for the sample application’s data, such as a Star Wars API or a data collection about World Cup contests? Esther Schindler, slashdot user #16185, assembled a groovy collection of datasets that may be useful but also may be a source of fascinating internet rabbit holes.