OsiriX for Mac OS is no longer really free, but there is a FOSS version called Horos (www.horosproject.org) that is maintained that started from the last open source version of OsiriX. If you need more granular access to the data, the (non-free) MATLAB Image Processing Toolbox provides reasonably good support for DICOM data.
It is easy to use, can load patient scans directly from CDs/DVDs or your computer, includes all the basic 3D rendering capabilities you would expect, as a great side-by-side comparison view, supports DICOM anonymization for HIPAA compliance, and more.