- The [UCI Machine Learning Repository][1] provides databases mostly aimed at machine learning researchers. 
 - [Amazon Public Data Sets][2] lists all public data sets that can be seamlessly integrated into AWS cloud-based application (currently 55 data sets are available).

Also, this [Quora thread][3] lists quite a few others.


  [1]: http://archive.ics.uci.edu/ml/
  [2]: http://aws.amazon.com/datasets
  [3]: https://www.quora.com/Data/Where-can-I-find-large-datasets-open-to-the-public