Classically Modern by Antony Walls (with additional contributions from the WoTC d20 Modern message boards) is a comprehensive set of documents based on the d20 Modern and D&D System Reference Documents. What they set out to do is provide a version of the d20 Modern system with classes, occupations and rules for as many settings as possible, including any combination of:
The documents cover low-magic Base, Advanced and Prestige Classes as well as F/X-based classes for Magic and Psionic settings. As the classes and occupations are clearly separated into eras of play, this is the perfect engine for multi-genre play. In short, if you’re looking for a d20-based system to replace GURPS, this is it. Add in the d20 Modern SRD for combat rules and monsters, and you’re done.
Classically Modern has become increasingly difficult to obtain online. The original domain hosting the site – www.merlinsworkshop.co.uk – is now a spamsite (don’t go there!), and any information about this system on other sites is either very out of date or links back to that domain.
In the spirit of high praise, I’ve put a zipfile of the collected pdfs online for download here (11Mb). If anyone can provide a more recent release or link to an official site, I’ll gladly revise and link there.
UPDATE: The files are available for direct download here.