In 1986, DC Comics did something that (at the time) was unprecedented. After almost fifty years of continuity, fifty years of characters and events and multiple realities, the editors at DC decided it was time to clean things up a bit. With the Crisis on Infinite Earths miniseries, the very first company-wide crossover in DC Comics history, and only the second one in the history of all comics (Marvel Secret Wars was first two years earlier), fifty years of continuity was wiped completely clean (unless you're Batman, Green Lantern or The Flash). The DC Multiverse was demolished in favor of one, stream-lined, DC Universe. It was something that had never been done before, and it succeeded in doing what it was meant to do. It breathed new life into Superman, it allowed Jason Todd to move away from being a carbon copy of Dick Grayson and allowed him to be a Robin of his own (and a Robin the fans wanted dead). It changed comics forever. Unfortunately, it would be repeated a couple of decades later to the point where the word Crisis is almost becoming cause for facepalm.
In 2006, DC Comics decided to do Infinite Crisis, which they followed two years later with Final Crisis, which turned out to be false advertising because three years after that they did Flashpoint, which, while it doesn't have the word Crisis in the name, it was a Crisis, it did everything a Crisis is supposed to do, and if you think it wasn't a Crisis, you're fooling yourself. Flashpoint was a Crisis event. And it was the third Crisis event in five years. When DC Comics was promoting Infinite Crisis, they were advertising it as the long-awaited sequel to Crisis on Infinite Earths. It was a direct sequel to that story, yes, but it wasn't the first Crisis event since 1986, like Dan DiDio would have had you believe. No, there was another Crisis. A Crisis in 1994. A Crisis in Time...
When the original Crisis was done, it was said that this was something that should be done every ten to fifteen years to keep the continuity clean. Eight years later, we got Zero Hour, literally subtitled the Crisis in Time. This was the true first sequel to the original Crisis. Hal Jordan, having just gone rogue from the Green Lantern Corps and now calling himself Parallax (this was before the stupid Geoff Johns retcons, when Parallax actually was Hal Jordan and not some yellow moster that had possessed Hal), and he was going to reorder the universe to correct what he felt were wrongs that needed righted. Zero Hour was a little bit different than the original Crisis, in that it wasn't meant to tear the DCU completely down and then build it back up, it was meant just to clean stuff up a bit, and like its predecessor, it was successful to various degrees in that endeavor. Some characters got major overhauls, Guy Gardner was changed from a Green Lantern wannabe using Sinestro's yellow ring to a shapeshifter called Warrior, who could actually grow weapons out of his body. It wasn't as prolific as the original by any means, but nothing will ever be as prolific as the original Crisis ever again. All in all, Zero Hour was a good read and it cleaned up some continuity errors left behind by the original Crisis.
Zero Hour was quite a bit shorter than the original Crisis (less than half the length at five issues), but it wasn't tasked with the enormous job that the original Crisis had either. One of the most interesting things about Zero Hour was that it begins in issue 4 and counts down to issue 0. It was written and illustrated by Dan Jurgens, and his artwork is superb as usual, if you're a fan of Dan's. If you're not a fan, and I can't imagine why anyone wouldn't be, well, then I guess the artwork is just as crappy as always (and I can't believe that sentence came out of my keyboard). A couple of new characters came out of this particular Crisis, Impulse and Damage were both introduced, and it carried on what would turn out to be a Crisis tradition at DC, it killed off a Flash. Now, of course, Wally West wasn't actually dead, but he did appear to have been absorbed into the speed force the same as Barry Allen had during the original Crisis. Wally is revealed to have been pulled into the timestream, not the speed force, and he returns immediately after the even in The Flash #0.
Zero Hour, being about a time crisis as opposed to just a universe crisis, had an opportunity that no Crisis event before or since has had. It was able to involve characters that were dead or had undergone severe changes, because it was them from the past brought into the present. Barbara Gordon got to be Batgirl again and we got to see several different incarnations of Hawkman, before DC realigned all of them into what was going to be the current version of the character. This was a simpler time in comics, which in retrospect seems like kind of a weird thing to say. I hate what DC has done to the the word Crisis in recent times, because they have run it into the ground we missed a golden opportunity to see what would have happened had they left it as an every 10-15 year event. I was really excited for Infinite Crisis, because it was time for another Crisis. I rolled my eyes at Final Crisis and by the time of Flashpoint, I never wanted to hear the words Crisis or reboot again. And this year, just four years after the last Crisis event, we are getting another Crisis event with Convergence. I really hope they get their act together soon, because pretty soon what was once a novel and interesting way of cleaning up continuity is going to become cliched and laughable, if it hasn't become that already.