MapWindow GIS is a free desktop geographic information system (GIS) application with an extensible plug-in architecture. It also includes a GIS ActiveX control and a fully C# GIS application programmer’s interface (API) called DotSpatial that can be used by software developers. MapWindow can be used as an open-source alternative to many commercial desktop GIS software packages, it can be used to distribute data to others who may not have a GIS, and it can be used to develop and distribute custom geospatial data analysis tools, or plug-ins.