No registration required

Erlangen Volume Library - diverse datasets, including DTI.
Dr Gordon Kindlmann's brain - high quality DTI dataset of Dr Kindlmann's brain, in NRRD format.
OASIS - The Open Access Structural Imaging Series (OASIS): starting with 400 brain datasets.
Osirix DICOM datasets - a rich collection of high resolution radiology datasets all in DICOM format.
University of Iowa Radiology department - a collection of slightly older but still usable datasets in Analyze format.
Utah SCI CT datasets archive - collection of CT datasets, including micro-CT, at the Utah Scientific Computing and Imaging Institute
VolVis.org dataset archive - collection of miscellaneous datasets, mostly in RAW format, focused on volume visualisation.

Registration required

National Cancer Imaging Archive - amongst other things, a CT colonography collection of 827 cases with same-day optical colonography.