Mapnik is described as 'Free Toolkit for developing mapping applications. Above all Mapnik is about making beautiful maps. It is easily extensible and suitable for both desktop and web development. - C++/Python GIS toolkit'. There are more than 10 alternatives to Mapnik for a variety of platforms, including Windows, Mac, Linux, Online / Web-based and Android. The best alternative is QGIS, which is both free and Open Source. Other great apps like Mapnik are GRASS GIS (Free, Open Source), GeoServer (Free, Open Source), what3words (Free) and Mapserver (Free, Open Source).
Spatial Feature Server by MapET is an ESRI Geoservices REST Implementation and works with all ESRI Rest API's. The source of published data is the spatial extension of the major RDBMS, such Oracle, Microsoft SQLServer, MySQL, PostgreSQL.