Horos is described as '- a free, open medical image viewer. The goal of the Horos Project is to develop a fully functional, 64-bit DICOM viewer for OS X based on OsiriX'. There are more than 25 alternatives to Horos for a variety of platforms, including Windows, Mac, Linux, Online / Web-based and iPhone. The best alternative is OsiriX. It's not free, so if you're looking for a free alternative, you could try RadiAnt DICOM Viewer or MicroDicom. Other great apps like Horos are Weasis (Free, Open Source), 3D Slicer (Free, Open Source), Miele-LXIV (Free, Open Source) and 3DimViewer (Free, Open Source).