If you're in the medical profession, then you'll know that DICOM (Digital Imaging and Communications in Medicine) is the industry standard to store, print and send images.
MiView is a free open source program which displays DICOM, ACR, NEMA and PAPYRUS, formats. The main aim of MIView is to provide a platform that allows you to view any kind of medical image and perform basic manipulation of them. The program does require a decent graphics card because it has to handle images that support multiple shading depths plus it needs to be able to render images render images in real time. To be honest, unless you are needing to view XRays or technical medical documents, you won't really find much use for this viewer as simple and effective as it is. It's also prone to several bugs including some images do not display properly (by the developer's own admission) and the "Multislice" view does not work correctly. In addition, it's known that the viewing area is transparent upon initial startup on some ATI cards.
Nevertheless, if you're a medical professional on a budget, this could be the viewer you've been looking for.