GURPS is great for resource mining. Their Space book, for instance, has real world mathematics in it (and the tables based on it) for creating entire solar systems.
3.5e is probably the most versatile in terms of sheer mass of third party stuff. Not all of it is great, but there is a skewed ratio to
good material. Books like Masterwork Maps: Inns and Taverns (it's B&W unfortunately, but someone with a set of PrismaColor Verithins could lightly touch it up and turn it into a masterpiece), or
Tournaments, Fairs and Taverns found
here. There's others, too, like Mongoose Publishing's
Ultimate Game Designer's Companion, which has rules for things like aerial combat, naval combat, chariot racing, and doing things with panache to earn reputation, as well as nitty-gritty combat that is much more lethal and new skills and feats as well as new uses for old skills. Absolutely gorgeous book that I cannot recommend highly enough. There's are literally thousands of books and publications online for 3.5, not to mention all the things like Dragon magazine, or it's companion publication, Dungeon. So much goodness in those, too.
Now, if you
really want detail (and are a secret masochist for the cerebral), check out the old school system Friday Night Firefight/Interlok that was the basis for the old Cyberpunk. All kinds of random cool things I wish they'd have ported over to 3.5e. There's sandbagging effects from high strength, for instance, that offers you DR. The body hit charts and DR system are far superior to armor class in that there is no armor class. Armor only makes you more durable against certain damage types. Only your high dexterity gives you a sort of "AC" as you are able to react faster. But there's also cool points you can spend on certain checks, and everything is a skill. Everything. Fire a gun? Skill. Want to jump up in the air, flip 720, and land on a nearby passing car, and you're blowing cool points on it like a scene straight out of
Big Trouble In Little China? That's a skill. lol Plus, there's an attempt point system and the higher your skill the greater your mastery. It's very lethal and unforgiving to the foolish.
There's also a lot of other resource books and whatnot, too, that can all be had--for
free--off DriveThruRPG.