MODFLOW
MODFLOW is the U.S. Geological Survey modular finite-difference flow model, which is a computer code that solves the groundwater flow equation. The program is used by hydrogeologists to simulate the flow of groundwater through aquifers. The source code is free public domain software, written primarily in Fortran, and can compile and run on Microsoft Windows or Unix-like operating systems. The compiled program is a command line tool that reads text file inputs that define the model grid, boundary conditions, and simulation time frame. Extensions beyond the command line program are several actively developed commercial and non-commercial graphical user interfaces.