grass - Are there any open source tools to detect and fill sinks on a DEM?

Are there any open source or inexpensive tools to detect and fill sinks on a DEM? ArcGIS Spatial Analyst is just out of my price range.

Answer

GRASS has r.fill.dir and better yet, r.terraflow, which is one of the few hydrology tools to work on massive rasters. There's also TauDem, which includes PitRemove for filling.