The non-profit International Neuroinformatics Coordinating Facility, INCF, has curated and provided links to several national and international datasets and repositories. These resources are meant to accelerate collaborative brain research but can be useful for other purposes, such as algorithm development and validation. The datasets are available on their website here.
XSEDE Science Gateways facilitate data-sharing among domain-based communities of practice. For a directory of National Science Foundation/XSEDE Gateways, visit their website.
If you are aware of other resources that you think should be mentioned, please contact us so that we can share those with others.