QGIS
FreeProfessional GIS that replaces a licence costing thousands a year
A complete open-source geographic information system. Load vector and raster data, run spatial analysis, process remote-sensing imagery and lay out publication-quality maps, with support for hundreds of formats and over a thousand plugins. Free for commercial use too — no licence fees, no dongles.
Why we picked it
GIS was long dominated by ArcGIS, whose commercial licence runs into thousands per year — out of reach for students and small organizations. QGIS is the field's most successful open-source project: 14,215 stars, GPL-2.0, feature coverage that handles the overwhelming majority of real work, and completely free for commercial use.
It loads Shapefiles, GeoJSON, GeoPackage and raster imagery, connects to PostGIS; runs buffer, overlay and network analysis plus terrain and slope calculations; processes and classifies satellite imagery; and finishes with a print layout engine that produces maps with legends, scale bars and north arrows. Archaeology, urban planning, ecology, hydrology and logistics siting all do their daily work in it.
The ecosystem is the other half of its strength. Over a thousand plugins cover everything from pulling OpenStreetMap data and connecting to online map services to specialist hydrological modelling and remote-sensing workflows. It also bundles the algorithms of GDAL, GRASS and SAGA, so one install brings an entire toolbox.
There is a learning curve, but it is not the software's fault — GIS as a discipline has a learning curve. Coordinate systems and projections are every beginner's first wall (data that appears to have "moved to Africa" is almost always a projection that was not set). The good news is that the official documentation and training manual are excellent.
One practical tip on versions: the site offers both the latest release and an LTR long-term support build. For production work that must stay stable, take the LTR; take the latest if you want new features.