Depends on the campaign we're playing.
For our "After Sunset" campaign set in a fictional California city in the present time, the base system is D20 Modern.
We use the After Sunset 3rd party books for some of the Vampire and Werewolf rules.
We take liberally from the White Wolf
Vampire and Werewolf games, and we take lots of inspiration from non-gaming sources, such as Blade, Buffy: The Vampire Slayer, Dracula 2000, American Werewolf in London, American Werewolf in Paris, The Howling, Underworld, Underworld: Evolution... etc... etc... etc...
Add in a smattering of D20 Future for my techie ubergeek, and you might get somewhat close to our game.
If we're going Post-Apocalyptic, then I take the D20 Modern base system, tack on several of the "Blood and _____" books from RPGObjects, a tiny bit of D20 Future, and a liberal helping of Year of the Zombie
, by UKG Publishing. Mix that all up for the rules, and then for resource/inspiration, I look to movies like the new Dawn of the Dead
, The Postman, Red Dawn,
and several others in that ilk...
For D20 Future campaign... that a real melting pot of resource material. D20 Future is the base rule system. I add in several 3rd party rulesets, and for resource material, I look everywhere... Starship Troopers
(movie), Stargate, Stargate: Atlantis, Wing Commander
(PC games), Starflight II, Battlestar Galactica, StarCraft, Aliens, Predator, Star Wars, Star Trek
(hehe... I commited the ultimate geek blasphemy... I had a group of Klingons flying TIE fighters performing pirate raids on backwater trade routes), Star Frontiers
(space RPG from the '80s), SpaceMaster
(futuristic RPG from Iron Crown Enterprises in the '80s), Buck Rogers...
If it's a Sci-Fi resource, you might find it in my game.