Mapnik is an open source mapping toolkit, written in C++ and can be scripted using binding languages such as JavaScript (Node.js), Python, Ruby, and Java. Useful for both Desktop- and Server-based map rendering. It is used by Tilemill, OpenStreetMap, Mapbox, CartoDB, Stamen, MapQuest and Kosmtik.
It's available on Linux, Mac, and Windows.
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