MRIcro is a medical image viewer for macOS. It can display popular medical imaging formats including DICOM and NIfTI. It is simple to use, in contrast to more powerful but necessarily more complicated tools. It supports statistical overlays, volume rendering, and 4D time series. It includes color schemes optimized for both MRI and CT scans.
BSD or BSD-style license: http://www.opensource.org/licenses/bsd-license.php
The BSD license is fairly permissive, placing few restrictions on the developer or user. As a developer, you may distribute source or binaries (with or without source code). As a tool user, you may modify and redistribute this tool or resource. If source code is distributed, a user may browse the source code to understand the tool's workings, modify the tool to better suit local needs, and share a modified version. If source code is not distributed, a user may still redistribute the tool as part of another package.