We present a catalog of XMM-Newton and Chandra observations of gamma-ray burst (GRB) afterglows, reduced in a common way using the most up-to-date calibration files and software. We focus on the continuum properties of the afterglows. We derive the spectral and temporal decay indices for 16 bursts. We place constraints on the burst environment and geometry. A comparison of the fast XMM-Newton follow-up and the late Chandra observations shows a significant difference in those parameters, likely produced by a transition from jet expansion taking place between two and ten days after the burst. We do not observe a significant shrinking of the luminosity distribution when we correct for beaming; more burst observations are needed to confirm this result. We also compare our results with those obtained by BeppoSAX and SWIFT; there is no strong discrepancy between the afterglow fluxes observed with these satellites when we carefully take into account the different median observation time of each observatory.