d20 Past is a splatbook for d20 Modern that, as its name suggests, focuses on running games set in the past, in contrast to the opposing splatbook d20 Future. Unlike that book its contents were never released under OGL.
It introduced the d20 Modern versions of the Sorcerer and Druid (Here named Shaman) with its many exclusive spells. It introduced several entirely new advanced classes and three new prestige classes, the most notable of which are Scientist, which can make inventions that replicate spell effects and Mesmerist, a strictly worse version of the core Telepath that's broken in the Truenamer sense on account of never actually saying what powers it can learn.
Only three new feats are added, as most of that section is spent outlining how lower tech levels interact. Minions and Sidekick are Leadership broken into two feats as it should have been ages ago. Minions is actually more broken than it normally is since a character's reputation modifier (easily increased) adds to leadership score and, if paired with Urban Arcana, minions can be used to lower the cost of rituals. Secret Identity gives an alternate identity and "people won’t connect your two identities unless confronted with incontrovertible evidence that you’re one person.", making it a predecessor of Vigilante.
New items include early firearms, both pre-cartridge and World Wars era firearms. Derringers are priced low enough that, by d20 Modern's strange wealth rules, you can buy an unlimited number of them. A lot of early vehicles are including planes, trains, automobiles and sailing ships. A few new sets of armor and various melee weapons are introduced, mostly archaic armor for those crazy enough to use d20 Modern to run a dungeon crawl campaign or, more likely, use with Shadowkind from Urban Arcana.
The book provides a list of introduction dates for various items. While the premise and broad scope is useful, it has several obvious errors like the Tommy Gun being the first individual firearm capable of automatic fire (Proceeded by the 1916 use of the Villar Perosa as an infantry weapon, and 1918's MP18 and BAR).