MRIcron is a nifti visualization program that we used for 20 years. It ccan quickly display nifit images, and also has a VOI drawing tool, and a dicom to nifti converter (dcm2nii). As of 2020, the development has shifted to MRIcroGL.
These programs are written by Chris Rorden - Paul swapped emails with him and working on some testing together.
Notes from github