This high-resolution map by James W. Head, Maria Zuber and collaborators based on data from the NASA Lunar Reconnaissance Orbiter covers craters down to 20 km in size. Note the beautiful video! Passed on by Amanda. Published in Science.
From their abstract they conclude, “The characteristics of pre- and postmare crater populations support the hypothesis that there were two populations of impactors in early solar system history and that the transition occurred near the time of the Orientale Basin event.”