The PIHM model is an open source hydrologic model based on the semi-discrete finite volume method. As its name implies, it was developed at Penn State University. An interesting characteristic of this model is that its interface is the open source GIS program QGIS. The project lead for PIHM is Chris Duffy who credits his involvement in the CCMP for motivating the development of the model. Initial funding to develop PIHM came from NSF, NOAA, and NASA. The model has been applied in multiple locations in Pennsylvania as well as in Maryland, Massachusetts, Delaware, and California. Internationally, it has been used in Greece, United Kingdom, Czech Republic, and Switzerland.
You can read more about the PIHM model and download the source code here – www.pihm.psu.edu. You can download QGIS here – www.qgis.org/en/site/