Another AlternateHistory.com work, The End, And Afterwards, by Andy C, builds up to the destruction of a good chunk of the planet, after an unmanned probe mysteriously goes haywire and ends up crashing into the Indian Ocean just off the coast of Madagascar, thus prompting the evacuation of as many of the survivors of humanity as possible.
"End Times" begins three months after a messy virus (that somehow involved gross swelling) wipes out most of the population past their mid-twenties.
Several stories on Everything2, including (but not limited to):
** MAJOR SPOILERS** Actually, we learn in later chapters (after Earth is sealed off from the rest of the universe by a Black Hole, much like a piece of a twisted balloon animal,) that humanity suffers a Crash EVERY FEW CENTURIES. Just when humanity is on the verge of discovering subatomic theory, everyone on the face of the planet has his or her memories and technological knowledge wiped in an instant. Surrounded by working vehicles, factories, and skyscrapers, civilization recovers pretty quickly. It turns out this is the plan of two protagonists to keep humanity from rendering Earth uninhabitable via nuclear war.
In The Gamer's Alliance, all the stories have been set some time after the most recent catastrophic event that has taken place in the world. The most recent story arc takes place over a decade after the Cataclysm which shattered continents, gave rise to the terrifying Godslayer and allowed demon hordes to invade the world en masse.
The AlternateHistory.com thread Protect and Survive is set in Newcastle-upon-Tyne, England, after a nuclear war between the Soviet Union and the West. Newcastle is somewhat spared, but suffers in the aftermath of the nuclear war.
Rosais set after humans left cyborgs to fix the ecosystem they destroyed by causing all natural life to disappear.
SCP-419 ("Window to the World"). SCP-419 is a window to another reality that looks upon a cityscape. At some point a war occurred and the city was devastated, with buildings burning and reduced to rubble.
S. D. Locke's proposal ("When Day Breaks"). The premise of this SCP-001 proposal is the Sun becoming anomalous. Any sunlight that comes into contact with organic matter effectively turns it into invincible, living goo with an urge to infect others, quickly turning most of the world's population into puddles. The tale itself chronicles the researchers and agents holding out for as long as they can, and ends with a horrific poem that evokes a mental image of Body Horror.
The main premise of the Halo based machinima "Shadows Of The Past" is a nuclear war took place in the time frame of 1970-2020. Based on the fact the characters are always referencing pop culture of this time period. This could also be Rule of Funny This is Played for Laughs since it's a comedy series.
U Realms Live: Every Campaign (save for the 2nd quarter of Senate of Deadlantis) takes place after a catastrophic event known as the Birth of Magic: The Sun Dragon Phanto died and turned into the moon, removing the immortality of the Elves who then proceeded to slaughter each other. 90% of the original Elves died. On top of that, Phanto's death corrupted his 6 Children (Ouro'ras, Quintara Lotus, Yvander, Vlarunga, Rokesh and Golestandt) into being the bearers of the 6 major schools of Magic, and these Magics allowed the Dragons to be able to create new races, which they did. Also, Gnomes appeared out of no where, and no one to this day knows why.