Arc Hydro
Esri's free hydrology framework for ArcGIS Pro — terrain preprocessing and the data model behind many agency workflows.
The global hydrography reference — basins, river networks, lakes and hydro-environmental attributes, free.
by WWF / HydroSHEDS consortium
HydroSHEDS is the dataset family that global water analysis is built on: hydrologically conditioned DEM derivatives, the HydroBASINS nested basin hierarchy, HydroRIVERS river networks, HydroLAKES, and the HydroATLAS attribute compendium that attaches hundreds of hydro-environmental variables to every basin and reach. Led by WWF with academic partners, it is freely available for commercial and non-commercial use alike.
For engineers it answers the recurring "what's upstream" questions at screening scale — basin boundaries for a desk study, river networks for routing setups, lake volumes for water balance — anywhere on earth, in minutes. (Note the naming coincidence only: this directory's working title has nothing to do with it.)
Ranked by the topics, categories and workflow stages they share with HydroSHEDS.
Esri's free hydrology framework for ArcGIS Pro — terrain preprocessing and the data model behind many agency workflows.
Four decades of geospatial engineering — r.watershed and the most battle-tested hydrology modules in GIS.

The geomorphometry powerhouse — hundreds of terrain and hydrology modules behind a free desktop GIS.

500+ fast geospatial analysis tools with the best DEM hydrology toolkit in open source.
Utah State's parallel terrain-analysis suite — D-infinity flow modelling and objective stream-network extraction.
The free, open-source GIS that has become the default spatial platform for water engineers worldwide.