It is fairly easy to create 3D buildings from OpenStreetMap (OSM) maps, essentially by extruding the 2D footprint polygons to 3D.

For the procedure using OSGeo software, see

https://grass.osgeo.org/wiki/Extrude_2D_polygons_to_3D

A first release candidate of GRASS 6.4.2 is now available.

Source code:
 https://grass.osgeo.org/grass64/source/
 https://grass.osgeo.org/grass64/source/grass-6.4.2RC1.tar.gz

winGRASS binaries:
  https://grass.osgeo.org/grass64/binary/mswindows/native/

To get the RC1 source code from SVN:
 svn checkout https://svn.osgeo.org/grass/grass/tags/release_20111010_grass_6_4_2RC1/

An announcement has been drafted at
 https://trac.osgeo.org/grass/wiki/Release/6.4.2RC1-News

Key improvements of the GRASS 6.4.2 release include enhanced portability for MS-Windows (native support), major fixes for the new wxPython based portable graphical interface, and new functionality (new network tools, zonal support for univariate statistics, image processing improvements, GRASS Python Library extensions, an embedded interactive Python Shell, an experimental Graphical Modeler, and new Cartographic Composer front-end to ps.map).

Release candidate management at
 https://trac.osgeo.org/grass/wiki/Grass6Planning

Please join us in testing this release candidate for the final release.

Thanks to all contributors!