1. QGIS (Quantum GIS)

• Platform: Windows, Mac, Linux
• What It Does: Full-featured GIS platform for mapping, analysis, data editing, and visualization.
• Why Use It: Active community, frequent updates, plugins, supports raster/vector/GPS data.
• Website: https://qgis.org

2. GRASS GIS

• Platform: Windows, Mac, Linux
• What It Does: Advanced geospatial analysis and modeling (terrain, hydrology, etc.)
• Best For: Scientific research and complex spatial modeling.
• Website: https://grass.osgeo.org

3. gvSIG

• Platform: Windows, Linux
• What It Does: Desktop GIS with CAD-like tools and 3D capabilities.
• Best For: Urban planning, environmental studies.
• Website: https://www.gvsig.com

4. MapServer

• Platform: Web-based
• What It Does: Publishes maps and geospatial data on the web (OGC-compliant).
• Best For: Building custom online mapping applications.
• Website: https://mapserver.org

5. GeoServer

• Platform: Web-based (Java-based)
• What It Does: Server for sharing spatial data using open standards (WMS, WFS, WCS).
• Best For: Web GIS applications, enterprise-level spatial data sharing.
• Website: https://geoserver.org

6. OpenLayers

• Platform: JavaScript Library
• What It Does: Build interactive web maps using open data layers (like OSM, WMS, GeoJSON).
• Best For: Developers creating browser-based GIS apps.
• Website: https://openlayers.org

7. Leaflet

• Platform: JavaScript Library
• What It Does: Lightweight web mapping with plugins and mobile support.
• Best For: Simple, interactive web maps (fast and easy).
• Website: https://leafletjs.com

8. SAGA GIS

• Platform: Windows, Linux
• What It Does: Terrain analysis, hydrology, geostatistics, and raster processing.
• Best For: Raster-heavy analysis and workflows.
• Website: http://www.saga-gis.org

9. uDig (User-Friendly Desktop Internet GIS)

• Platform: Windows, Mac, Linux
• What It Does: Basic mapping and editing with integration for web services.
• Best For: Lightweight GIS tasks.
• Website: http://udig.refractions.net

10. WhiteboxTools / Whitebox GAT

• Platform: Desktop and Python API
• What It Does: Terrain analysis, remote sensing, hydrological modeling.
• Best For: Researchers and advanced users.
• Website: https://www.whiteboxgeo.com

Bonus: Free Geospatial Data Sources

• OpenStreetMap (OSM) – https://www.openstreetmap.org
• USGS Earth Explorer – https://earthexplorer.usgs.gov
• Natural Earth Data – https://www.naturalearthdata.com