  In the set of 236 GRB afterglows observed by Swift between January 2005 and
March 2007, we identify 30 X-ray light-curves whose power-law fall-off exhibit
a steepening ("break") at 0.1-10 day after trigger, to a decay steeper than
t^{-1.5}. For most of these afterglows, the X-ray spectral slope and the decay
indices before and after the break can be accommodated by the standard jet
model although a different origin of the breaks cannot be ruled out. In
addition, there are 27 other afterglows whose X-ray light-curves may also
exhibit a late break to a steep decay, but the evidence is not that compelling.
  The X-ray emissions of 38 afterglows decay slower than t^{-1.5} until after 3
day, half of them exhibiting such a slow decay until after 10 day.
  Therefore, the fraction of well-monitored Swift afterglows with potential
jet-breaks is around 60 percent, whether we count only the strongest cases for
each type or all of them. This fraction is comparable to the 75 percent of
pre-Swift afterglows whose optical light-curves displayed similar breaks at ~1
day.
  The properties of the prompt emission of Swift afterglows with light-curve
breaks show the same correlations (peak energy of GRB spectrum with the burst
isotropic output and with burst collimated output) as previously found for
pre-Swift optical afterglows with light-curve breaks (the Amati and Ghirlanda
relations, respectively). However, we find that Ghirlanda relation is largely a
consequence of Amati's and that the use of the jet-break time leads to a
stronger Ghirlanda correlation only when the few outliers to the Amati relation
are included.
