You'll find a ton of public datasets on GitHub [1].
Maven Analytics offers a monthly data analytics challenge [2] that you can enter for free. See their past competitions for some interesting datasets.
As I'm based in Ireland I'll also recommend the Irish Data Portal [3].
[1] https://github.com/awesomedata/awesome-public-datasets [2] https://mavenanalytics.io/challenges [3] https://data.gov.ie/