iMOD (since 2006) is an easy to use Graphical User Interface + an accelerated Deltares-version of MODFLOW with fast, flexible and consistent sub-domain modeling techniques.

iMOD facilitates very large, high resolution MODFLOW groundwater modeling and geo-editing of the subsurface. 
Key features of iMOD are:

• One expandable data set covering all possible future areas of interest
• Accelerated modelling source codes to enable fast, flexible and consistent sub-domain modelling techniques
• Flow model nesting, toggling between grid resolutions and moving to new areas of interest
• Efficient numerical modelling
• Fast interactive 2D- and 3D-analysis and visualisation
• Interactively editing the geometry of the subsurface
• Consistency between regional and sub-domain models
• Leaving the era of building series of individual models behind
• Modelling of water quantity and water quality (iMOD WQ)
• Support creating of (nested) MODFLOW 6 models

iMOD is open source since June 2014.
The Fortran source code of the iMOD GUI and the accelerated Deltares-version of MODFLOW is available. You can include your own features and re-compile your executable (Winteracter and OpenGL libraries required).

Click here to download iMOD 5