When you mention QGIS, you should also mention GDAL, JTS, udig, geoserver, open stree maps, open scene graph, FWtools etc. Open source GIS has awesome list of projects and people, QGIS being only one of them. It really fascinates me.
I think it did to GIS what Sagemath did to free/open source mathematical software. It integrated everything in a nice package, freeing the users from the burden of dealing with countless disparate packages.
Yeah out of these GDAL is the one I use the most because it's easy to script. I honestly find it easier to use GDAL on the command line than the QGIS GUI which says a lot about the latter.
Also PROJ, the quiet bit closer to the core that does projections.
The sqlite db alone that's packaged with PROJ is a pillar of knowledge that one can only marvel at. The most authoritative and wide-ranging collection of projection/datum information I'm aware of.
