We use SAGA GIS for raster related tasks, especially related to geomorphometry / terrain analysis. For instance, SAGA can calculate geomorphons, openness, and many types of curvature not available in ArcGIS Pro. It can also compute many geostatistics, perform image analysis, and support standard GIS operations like reprojection, data import and export, and standard vector and raster tools. It features a Python API, but this appears to be outdated.

Dr. Pingel has prepared a Tutorial Video on SAGA (15 mins).

• Download the SAGA zipfile from sourceforge.net/projects/saga-gis, and unzip it.
• Open saga_gui.exe
• Open a raster with File -> Open. The layer will populate to the Manager -> Data Pane, but will not draw.
• To render it, right click and select Add to Map.
• Open the Geoprocessing tool you want to use. Most of the ones we use in the lab are in Terrain Analysis, but there are a host of other options available as well. You must set the grid system and inputs by selecting the layer you want to operate on. You'll likely want to the output layer. Set any desired parameters. A status bar is in the lower right hand corner. The output will not draw, but will instead populate to your Data pane. Right click and Add to Map to draw it.
• Two other handy features are:
  • SAGA can also create a 3D view (Map -> 3D View), and includes easily adjustable vertical exaggeration (F1 and F2 keys).
  • You can turn on "Show Cell Values" to see the original values in the raster. Genius!
• To Export data, just right click on your data layer, and export to a GeoTiff.

• SAGA GIS Wikipedia Page
• Sourceforge Page
• Conrad et al. 2015. System for Automated Geoscientific Analyses (SAGA) v. 2.1.4.