Currently I'm beginning a 3rd Edition campaign that's set in the universe of Stargate SG-1. I imported rules from D20 Modern and the short-lived Stargate RPG to cover the non-medieval stuff. They're all based on the D20 system, so you can do that relatively easily. (Obviously, most of my game is custom.)
Just had to report that my customized Stargate game is going AWESOMELY.
I took the 3.5 Edition Players Handbook and rewrote sections of it to accommodate non-medieval characters. The D20 Modern books establish levels of civilization, Progress Levels. Medieval/D&D characters are PL 2. Modern-day characters are PL 5. TNG-type characters are PL 7. Since any kind of world is available in Stargate, I had the players pick which PL their character was from.
I imported a bunch of new skills like Craft (Mechanical), Computer Use, etc, and then established starting skills for each PL. Lower PLs get to take things like Handle Animal and Survival. Higher PLs get things like Electronics and Computer Use, but they do NOT get Handle Animal or Survival, at least initially.
I also came up with some customized classes for modern-type characters, like "gadgeter" and "technologist". Currently the group consists of two rangers, a monk, a bard, a rogue, a gadgeter, a "naturomedic", a "daredevil", a "transchemic", and a psionic. PLs ranging from 2 to 8.
Then I explained how all the characters came to be in the same place, which was they were prisoners in the dungeon of a Goa'uld System Lord, and that they were in the middle of a breakout. "What do you do?"
Currently the group is jumping from world to world, trying to find a place where they aren't hunted by the Goa'uld. They have such a wide range that I can throw just about any type of challenge at them, and one member of the group will be able to step forward and have a chance to shine. Everyone is having a blast.
Just had to boast a little.