d20 Apocalypse is a splatbook for d20 Modern that is a spin-off to d20 Future. As its name suggests, it is focused on running post-apocalyptic games, with multiple different kinds of post-apocalyptic settings being presented for DMs to use.
This book finally introduced an alternative to d20 Modern's bizarre wealth rules in the form of Trade Value, a system of "worth" and unexploited trading items. Sadly the values of this system is weighted toward apocalyptic settings, and very specific kinds of those, with individual rounds of ammo being high priced.
The first chapter contains the following Apocalypse types:
- Alien Invasion
- Asteroid Strike
- Biological Disaster
- Environmental Cataclysm
- Judgment Day
- Nuclear Armageddon
- Rise of the Machines
- Rogue Planet
- Supernatural Invasion
There's also suggestions for mixing different kinds of apocalypse as well. The second half of the book is devoted to three sample post-apocalyptic campaign settings inspired by this list.
This is then followed by a basic summary of different kinds of postapocalyptic societies, the distinct post-apocalyptic eras, how to involve the heroes, and new mechanics in the form of Devastation and Radioactivity.
Of the 4 Advanced Classes included, two of them (Salvager and Road Warrior) are are considered to make up some of the worst classes in the entire system as they are focused on a single thing while not being very good at it. If your GM actually follows the expected rewards chart, Lawbringer may not be that much better since its main ability of getting 80 extra Trade Value over 5 levels is worth less than a single encounter's worth of salvage by the time they get it. On the bright side, Evolutionary is considered one of the best, being both flavorful and mechanically useful with its ability to select mutations at level up and losing negative ones.
The first of the three postapocalyptic worlds featured in this book, Earth Inherited takes place on a world where the Rapture occurred; the vast bulk of humanity was whisked away to Heaven or Hell, and the survivors were left behind as armies of angels and fiends flooded the world to begin battling it out for pretty much their own reasons.
What gives the survivors a fighting chance is that this world had been futuristic enough to develop robots and power armor, so you have gritty human survivors in mecha gunning down angels and fiends alike for refusing to stop blowing up their planet.
Atomic Sunrise is the obligatory post-nuclear war setting, where the world was blown to hell in a firestorm of atomic energies and now the survivors have to do the best they can in a scarred, mutation-riddled world overrun by marauders and desperados.
Plague World takes place on an earth that was invaded by a race of aliens called the Spanthi, with the resultant war being fought to almost complete mutual annihilation. Set three hundred years after the start of the war, the alien invaders have degenerated into mutated killing machines, whilst war-spawned diseases, mutations and poisonous weapons flock a ravaged earth.
In this setting, the default assumption is that the party is playing a Rip Van team; an elite band of agents placed in suspended animation to be deployed to reclaim the world for humanity after the departure of the alien ships.