OpenSceneGraph
OpenSceneGraph can read directly from the GRASS database if the GRASS-GDAL plugin is installed.
Example:
GRASSDATA=$HOME/grassdata/nc_spm_08/PERMANENT
export GRASSDATA
osgdem --xx 10 --yy 10 -t ${GRASSDATA}/cellhd/landuse96_28m \
--xx 10 --yy 10 -d ${GRASSDATA}/cellhd/elevation \
-l 4 -v 0.1 -o north_carolina.ive
# you can already watch while the conversion is still running:
osgviewer north_carolina.ive