OSGeo4W includes open source GIS desktop applications (QGIS, GRASS GIS), geospatial libraries (PROJ, GDAL/OGR, GEOS, SpatiaLite, SAGA GIS), scripting languages (Python) as well as many other geospatial related packages.
The OSGeo4W installer bundles relevant OSGeo software packages plus additional requirements into a convenient MS Windows based installer. Re-running the installer will update existing packages and/or permit to modify the installation.
The Express install is a quick way of installing some of the most high profile OSGeo4W packages in a minimum number of steps. The Advanced install allows control over many details of the installation such as where to install, use of proxies, and local cache directory selection.
OSGeo4W is a joint voluntary community effort under the umbrella of the Open Source Geospatial Foundation. Contributions for maintaining the environment are welcome.