As a stable release GRASS 6.4 will enjoy long term support. While active development has continued in the background, due to our highly conservative stabilization policy this is the first official release of GRASS to introduce new features since October 2006. It supersedes the previous stable GRASS 6.2 line while remaining backward-compatible with all previous releases since 6.0. As such there are a host of new features to explore and it incorporates many structural improvements.
GRASS 6.4 debuts our new and improved wxPython graphical user interface (wxGUI); Python is now a fully supported scripting language; and for the first time since its inception with a port from the VAX 11/780 in 1983, GRASS now runs natively on a non-UNIX based platform: MS-Windows. GRASS has been translated into twenty languages and supports a wide array of popular data formats.
GRASS differs from many other GIS software packages used in the academic and professional worlds in that it is developed and distributed by users for users, mostly on a volunteer basis, in the open, and is given away for free. The source code is also freely available, allowing for immediate customization, examination of the underlying algorithms, addition of new features, and faster patching of bugs.
Full story at http://grass.osgeo.org/announces/announce_grass640.html