  We build differential-reddening maps for 66 Galactic globular clusters (GCs)
with archival HST WFC/ACS F606W and F814W photometry. Because of the different
GC sizes (characterised by the half-light radius $R_h$) and distances to the
Sun, the WFC/ACS field of view ($200\arcsec\times200\arcsec$) coverage
($R_{obs}$) lies in the range $1\la R_{obs}/R_h\la15$ for about 85% of the
sample, with about 10% covering only the inner ($R_{obs}\la R_h$) parts. We
divide the WFC/ACS field of view across each cluster in a regular cell grid,
and extract the stellar-density Hess diagram from each cell, shifting it in
colour and magnitude along the reddening vector until matching the mean
diagram. Thus, the maps correspond to the internal dispersion of the reddening
around the mean. Depending on the number of available stars (i.e. probable
members with adequate photometric errors), the angular resolution of the maps
range from $\approx7\arcsec\times7\arcsec$ to
$\approx20\arcsec\times20\arcsec$. We detect spatially-variable extinction in
the 66 globular clusters studied, with mean values ranging from
$\mEBV\approx0.018$ (NGC\,6981) up to $\mEBV\approx0.16$ (Palomar\,2).
Differential-reddening correction decreases the observed foreground reddening
and the apparent distance modulus but, since they are related to the same value
of \EBV, the distance to the Sun is conserved. Fits to the mean-ridge lines of
the highly-extincted and photometrically scattered globular cluster Palomar\,2
show that age and metallicity also remain unchanged after the
differential-reddening correction, but measurement uncertainties decrease
because of the reduced scatter. The lack of systematic variations of \mEBV\
with both the foreground reddening and the sampled cluster area indicates that
the main source of differential reddening is interstellar.
