EPANET is software that models water distribution piping systems. EPANET is public domain software that may be freely copied and distributed. It is a Windows 95/98/NT/XP/7/8 program. EPANET performs extended period simulation of the water movement and quality behavior within pressurized pipe networks.
Although not EPANET specific, the AWWA publishes a great book (M32) on modeling drinking water system
EPACAD is a utility for turning an AutoCAD file into an EPANET file. You will still have to edit the file in EPANET to add pipe and junction properties as well as any other necessary network features.
gHydraulics is a plugin to QGIS and helps you create an EPANET file from GIS files. You may still have to edit features in EPANET before you will get a working model.