When I was small, Neverwinter Nights was very popular, and we'd look through the little game's manual, a lot.
Recently, Fallout and Elder Scrolls, the Bethesda open worlds have inspired me. Games like that are pretty great for inspiring a want for an RPG set in an open world of that kind of genre.
I think the most inspirational these days is Dwarf Fortress. Adventure Mode really seals it in for me and I find it very satisfying. It's, I think, the quintessential, player-driven sandbox. Are you going to ask some lord what business they'd like you to take care of? If you're not their hearth-person, do they even give a shit about you? If they ask you to go slay the bronze colossus, are you going to quickly get the hell out of that town and, instead, form a performing troupe and entertain the lord of some other hamlet because you don't feel particularly suicidal? It also scratches my OSR/rogue-like itch, because you can die in wonderfully unexpected ways. Once my Goblin Demi-God was killed in one blow from a crossbow bolt to the brain. It was magical.
That's just Adventure Mode, though. Fortress Mode presents its own intriguing possibilities and Legends Mode especially lets you focus in on all the incredible aspects of Dwarf Fortress. You could just have Dwarf Fortress make a world and then, in Legends Mode, use that as you're setting (and I believe that's been done before). I've been considering how I might scrap together a Dwarf Fortress RPG, actually. It might be something like GLOG or even Knave, but GURPS, especially with Martial Arts, is also very feasible.
Project Zomboid sometimes gives me bits of information, and Darkwood has some very inspirational imagery and ideas for horror and, perhaps, fantasy horror.
Oddly enough O.R.B. has inspired me a bit, so I'd like to perhaps make something for it in Traveller/Cepheus Engine. It has a pretty in-depth setting available in its manual and in-game.