TauDEM

TauDEM is a toolset for extracting hydrologic information from a digital elevation model: pit filling, D8 and D-infinity flow directions, contributing area, stream network delineation with an objective threshold, watershed and subwatershed delineation for each stream segment, the topographic wetness index and distances to streams and ridges. It reads rasters through GDAL, works in geographic coordinates too and splits the work across processors, so large DEMs can be handled. Limitation: without ArcGIS you work from the command line, though the tools can be scripted or combined with other GIS software.