Having switched from D&D to D20 Modern for a while, we noticed that many of the rules just don't seem to mesh properly, thus our mighty DM has rewritten a significant portion of the rules, including everything from the basic hero classes to vehicles, mecha and spaceships.
Feel free to download them and peruse them for yourself.
Constructive comments would be appreciated, and can be discussed with the author (AussieGamer) here