D20 Apocalypse
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 bizzare 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.