Circa After The Apocalypse takes place after an undetermined time period after Earth is bombarded by meteors, which devastate the world and raise a dust cloud that blots out the sun- the dust cloud seems to have settled prior to the events of the comic itself.
There are also a couple alternate universes where, while the most of humanity hasn't died, they have been transported into other dimensions, leaving very few people behind. In one there's no one around but an Alternate Universe Riff and six billion butterflies that replaced every single person on Earth.
Magical Misfits is set in the far future after magic returned & killed some humans while changing others into creatures of mythology.
Unicorn Jelly takes place in a strange pocket universe where the laws of physics are different from ours. Some time in the past one of the triangular "worldplates" where humans and natives lived was destroyed by something resembling an atomic bomb, resulting in a rain of debris that destroyed other worldplates, creating a chain reaction of destruction that will eventually obliterate every worldplate in this finite but unbounded cosmos. It's later discovered that new worldplates will gradually coalesce out of the debris.
FreakAngels is heavy with this. "London is drowning," seems to be the catchphrase here. Significant portions of Great Britain (and perhaps the world) are submerged underwater, and it turns out that the eponymous protagonists are to blame.
Bubble Wrap - where most of humanity was wiped out due to a contagious virus. The surviving humans don't have to wear protective suits at all times...unless they have an aversion to dying, in which case it's a good idea to stay dressed.
C 0 DA - The sequel to The Elder Scrolls series by longtime series mainstay Michael Kirkbride (with support from current Bethesda writer Kurt Kuhlmann). Numidium has made Landfall. The Thalmor won, time is unbound, Nirn is uninhabitable and the survivors eke out a living under the surface of the moon. Humans are ambiguously extinct, as the only races seen in the comic outside of flashbacks are Dunmer, Khajiit and Hist.
White Noise takes place 120 years after a hostile alien race temporarily halted the rotation of the Earth, killing everyone and everything not on one of the three space colonies.
Post Nuke takes place following a nuclear war that played out eerily similar to the biblical apocalypse. A mad dictator took over, nuclear fallout poisoned a third of all water and a third of all land, and there was very little warning of the first shots in the Last War being fired-people say it came like a thief in the night.
O31 is set in San Francisco in a world where all electricity has simply and mysteriously disappeared (which was followed promptly by gigantic explosions worldwide).
The Ends is set in a post-nuclear wasteland, but that's only the beginning of the wierdness. Apparently the nuclear apocalypse set up a "Groundhog Day" Loop, forcing its inhabitants to endure an endless cycle of death and rebirth.
The world's been "broken" four times in The Dragon Doctors. Once in a war between evil puppet masters and upstart magic users, a nuclear war, a meteor impact, and a disaster called "The Dimension Fusion" that mixed up parts of Earth with parts of other dimensions. Each time started a new calendar.
Soul To Call: Referred to as 'the Fall'. Two thirds of the human popular were wiped out, and malicious supernatural forces seem to have filled the void they've left…
The Order of the Stick is set on what turns out to be the second universe, after the first was destroyed by an Eldritch Abomination called the Snarl. All may not be as it appears, however, as it was recently revealed that the first universe may still exist.
What It Takes which takes place after most of civilization has died. Commonly referred to as "the flip."
Paonia Pawns visits various alternate worlds suffering through versions of this.
The Demon Archives is set on Earth about 100 years after nuclear and biological war destroy global civilization. A few high-tech organizations were prepared, survived the nuclear winter and actively work to re-establish civilization on the war-torn Earth.
Cthulhu Slippers is set after the end of the world as we know it after an invasion of all powerful alien gods. That the gods are dumber than a bag of hair is beside the point.
The prologue of Stand Still. Stay Silent showed the first few days after the discovery of a mysterious "rash illness" through the eyes of a several groups of Scandinavians who either got stuck in isolated areas or ran for the hills as the disease spread. The story itself picks up 90 years later with their decendants. Exactly what happened, and whether or not the character's beliefs that Trolls and Giants rule the world outside of Iceland and the few tiny enclaves they live in are true, is still unclear.
Only Human takes place 500 years after the end, with humanity as we know it gone.