There are two papers in the journal "Meteoritics" (which may have just changed its name to "Meteoritics and Planetary Science", I think) which provide information on impact crater ages, so that individuals can decide for themselves if there is periodicity. These are
Grieve, R. A. F., Meteoritics 26, 175 (1991) "Terrestial Impact: the record in the rocks", which contains a catalog of over 100 large craters on the earth, listing the location, diameters, and estimated age of each. This is a very nice overview! Grieve pours cold water on claims of periodicity: "... we have shown that, when actual age uncertainties are taken into account, it is essentially impossible to define any stable periodicity in the terrestial record ..." (see Grieve et al., Proc. Lunar Planet Sci. Conf. 18, p. 375 (1987)) Deutch, A., and Scharer, U., Meteoritics 29, 301 (1994) "Dating terrestial impact events," which describes the many subtleties involved in dating a crater. In a section titled "The Question of Periodicity in Cratering on the Earth," the authors write " ... the most serious objection against the hypothesis [that impacts are periodic] stems from the fact that the data base is insufficient for statistical analyses, because the crater ages used in the calculations included data of highly variable quality (some are definitely erroneous)."
Some other sites which may be of interest are (thanks to Paul V. Heinrich):