(Geographic Resources Analysis Support System)
GRASS is a free Opensource GIS produced by the USG for processing ALL kinds of data including the LIDAR and the NDVI analysis so beloved of the PrecisionAG movement.
Its been in use for at least the past decade and is quite well regarded in certain fields. Extensive documentation and tutorials are available at the website for same.
hzl
Comments
A nice QGIS feature is that it is easy to integrate features from GRASS, R, OTB and lots of other geospatial packages. In fact, many are integrated so well you don't even know you're using them. It's still worth learning those other packages on their own but that's no longer a requirement to benefit from their strengths.
Also using QGIS here - IMHO, easier for people without a PhD in geology / geography / survey engineers...
Yes, I've known it, but I too use QGIS most of the time. In ecology that package is slowly but surely overtaking ESRI, great integration with the also open source/free statistical package R.
I have it but find it just above my pay grade. It gets better all the time though.