

MapWindow GIS
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MapWindow is a fully free and open source geographic information system project. It includes an open source programming component, MapWinGIS ActiveX, and an extensible desktop application that supports plug-ins written in .NET (C# and VB.NET).
Comments and Reviews
Can program this using .Net and is/has the ability to parallelize lots of GIS operations. You can also extend its functionality with open source code.
There are a whole lot of bloated, incoherent, unusable, expensive far beyond the point of obscenity, and/or generally broken GIS utilities out there. MapWindow is not one of them. MWGIS + Shape2Earth plugin made converting ArcGIS shapefiles into georeferenced polylines in KML a task much less horrifying than a creepy-long-haired-asian-girl movie... quite the opposite of ArcGIS Explorer which shall be consumed utterly in the infernal flames of Perdition (if there is any justice in the universe).