The Second World Sourcebook will provide a bridge between d20 Fantasy and d20 Modern settings. Here's the blurb from the Second World webpage (
www.Second-World-Simulations.com):
Occasionally a person steps around the corner of an alley, finds a hole in the back of their closet, takes a shortcut down a wooded road at night, or simply turns their head at just the right moment to watch a falling leaf and then finds himself elsewhere. These people are called Exiles; they step or turn out of our world, the world of crowded freeways and glittering convenience stores and into the Second World, a place of magic, mystery, danger, and more than a little bit of terror. There are two worlds you see, separated by a roiling gulf of chaos called the Forge.
The Second World Sourcebook bridges the gap between modern day adventuring and traditional fantasy roleplaying. It provides a "campaign template" for running a dual-world game. By combining the sourcebook with your own favorite setting you can take your fantasy characters and run them through a modern day scenario, or take your modern characters and run them through one of the many fantasy modules currently available. Most modern settings already describe the impact of fantasy elements on them and include rules for handling magic. But what would happen if people from the modern world regularly trafficked with those in a more traditional fantasy world? What impact would it have if powerful wizards originally studied computer science at UCLA? What would the social institutions look like if the progress of ideas and politics we've seen in our world carried over to a place where electricity and gunpowder simply didn't work, but magic and divine power did?
One thing I think we lose sight of with high definition televisions, computers that double in power every 18 months or so, and medical technology that has progressed so rapidly that it outstripped the imaginations of science fiction writers from just 30 years ago, is that a more subtle kind of technology has progressed alongside the hardware and wetware we normally associate with scientific advance. Economic theory, the advantages and limitations of free market economies, the growth of corporate powers, the art of conventional and covert war, espionage, intelligence gathering, forensic science, the dangers of republics and the individual empowerment they provide, all these are no less technological advances than new ways to hurl lead and these sorts of technologies would spread no matter what your physics.
To answer these sorts of questions I include a rich system for dealing with social influence and 30-some organizations that use that system. I also include a cosmology of the world that explains why you'd have two parallel worlds of this sort and how to resolve interactions between them. I also include guidelines for how to handle characters from a modern setting when they translate to the fantasy setting, and vice versa. This should make it easier to run an occasional modern adventure with your regular fantasy characters or a fantasy adventure with your regular modern characters. Finally, I include a set of special prestige classes called Wardens; I make these as customizable as possible by including a menu of at least 25 abilities each that they choose from as they advance. In some ways they'll feel like the player-ready templates from my first product, "Bodies and Souls: Twenty Templates".