This page is dedicated to sparse and casual tricks with some common GFOSS tools.

This command works with relatively recent Gdal versions:

gdal_merge.py -o multiband.tif -separate band1.tif band2.tif band3.tif

Just use the builtin computing region facility, it will use a nearest neighbor interpolation for this purpose:

g.region rast=original_map -p
g.region res=<new>
r.mapcalc "new_map=original_map"

A similar trick can be used to cut a portion of the original map, apply a mask, etc. A more capable module for those applications is r.resamp.interp which can be used instead of r.mapcalc to peform bilinear and bicubic interpolation for the grid.

See here for details about using GRASS with national service at PCN.