I'd heard some suggest that mass shootings are more common now (in the post-Columbine era) in the U.S. than before; does anyone have some data as to whether this is so?

I have one seemingly reliable piece of data handy — the list in Gary Kleck's Targeting Guns, p. 144, reports on all the mass shootings Kleck knew of from 1984-1993, with mass shootings "defined here, somewhat arbitrarily, as an incident in which six or more victims were shot dead with a gun, or twelve or more total were wounded" (pp. 124-25). That list reports 15 mass killings, roughly evenly distributed from 1984 to 1993. (For those who want to check for completeness, the murderers are Ferguson, Ferri, Hennard, Doody & Garcia, Abeyta, Pugh, Wesbecker, Purdy, Farley, Simmons, Schnick, Cruse, Sherrill, Huberty, and Thomas.) My sense is that the frequency has not gone up materially since then, though I should note that this is just based on my likely quite faulty memory.

On the other hand, only one of those shootings (Purdy, in Stockton) was at a school, and it did not involve a student, unlike the Columbine murders and some of those that followed. My sense is that schoolyard shootings are indeed up since Columbine, but again I don't have handy data about how much. I'd also love to hear about data from before 1984; of course, Charles Whitman's murders in 1966, were at a university, but I do not know of any pattern of school or university mass shootings after that. (I would bracket the 1970 Kent State shootings, simply because they seem so radically different in motivation from the other killings that it's hard to see what sound policy analysis one could engage in that would group these shootings together with the other shootings I mention.)

UPDATE: A Better Where To Find has a long list, not claimed to be complete, of multiple-victim shootings, though with a somewhat different selection criterion than that given above, and limited to schools.