Distribution networks
EPANET engine is the most used engine in the world. Beyond the EPANET application itself, Qatium, InfoWater (Pro), and WaterGEMS - among others - use an EPANET engine. Even water modeling solutions with proprietary engines (like InfoWorks WS and Sinergee) can export to EPANET format.
Waste and stormwater networks
SWMM engine is the most used engine for waste and stormwater networks. Beyond the EPA SWMM application itself, InfoSWMM, PCSWMM - and many others - are using the SWMM engine. Even water modeling solutions with proprietary engines (like InfoWorks ICM) can export to the EPA SWMM format.