ADNI
Alzheimer's Disease Neuroimaging Initiative (ADNI) researchers collect several types of data from study volunteers throughout their participation in the study, using a standard set of protocols and procedures to eliminate inconsistencies.
They have different modalities of data such as clinical, genetic, MRI, PET, and Biospecimen.
This information is available for free to authorized investigators
OASIS
The Open Access Series of Imaging Studies (OASIS) is a project aimed at making neuroimaging data sets of the brain freely available to the scientific community.
The dataset contains over 2000 MR sessions. Many of the MR sessions are accompanied by volumetric segmentation files produced through Freesurfer processing. PET imaging from 3 different tracers, PIB, AV45, and FDG, totaling over 1500 raw imaging scans and the accompanying post-processed files from the Pet Unified Pipeline (PUP) are also available
By compiling and freely distributing neuroimaging data sets, they hope to facilitate future discoveries in basic and clinical neuroscience.