...
- ImageJ runs on Linux, Mac OS 9, Mac OS X, Windows, and the Sharp Zaurus PDA.
- FusionViewer open source & multiplatform PET/CT display software package
- OSIRIX (MAC OSX)
- AIM on ClearCanvas
- 3D Slicer
Some overviews to get you started with DICOM medical image data like those in TCIA include:
- I Do Imaging at http://idoimaging.com/ includes demonstrations, a wiki, and a blog for several open source medical imaging viewers.
- DICOM is Easy includes a detailed tutorial which begins at http://dicomiseasy.blogspot.com/p/introduction-to-dicom.html
- you can also search "DICOM tutorial" or DICOM overview" with your favorite search engine; there are many video walkthroughs on the Web as well.