A few more additions to the full list of free Mac GIS software, where they will be added shortly; you need only bookmark that preceding link to have it as a complete and regularly-updated reference.
Grid Analysis and Display System (GrADS) – “The Grid Analysis and Display System (GrADS) is an interactive desktop tool that is used for easy access, manipulation, and visualization of earth science data. The format of the data may be either binary, GRIB, NetCDF, or HDF-SDS (Scientific Data Sets).”
MARPLOT – “MARPLOT is the mapping application. It allows users to “see” their data (e.g., roads, facilities, schools, response assets), display this information on computer maps, and print the information on area maps. The areas contaminated by potential or actual chemical release scenarios also can be overlaid on the maps to determine potential impacts. The maps are created from the U.S. Bureau of Census TIGER/Line files and can be manipulated quickly to show possible hazard areas.”
Puzzle-GIS – “Puzzle is a Java Geographic Information Systems (GIS) development platform build on top of NetBeans6. The main goal of Puzzle GIS is to create a free, production ready and competitive tool as a valid alternative to commercial software.”
Spatial Commander – A free German GIS program. This link (in German) lists its features.
Spatial Data Integrator – “Spatial Data Integrator (SDI) is an ETL (Extract Transform Load) tool with geospatial capabilities.”
STEM – “The Spatiotemporal Epidemiological Modeler (STEM) tool is designed to help scientists and public health officials create and use spatial and temporal models of emerging infectious diseases. These models could aid in understanding, and potentially preventing, the spread such diseases.”