In my opinion, D20 Modern was a wonderful branch off the D&D 3.0 line.
Twisting the fantasy D20 SRD to modern or futuristic games just wasn't exciting
me. D20 Modern provided a ruleset tailor made for those non-fantasy
These rules and supplements are all very useful for the D20 Modern version of
Gamma World as well as Darwin's World. Also, some Polyhedron magazine
mini-games can add to the modern fun.
D20 Modern SRD
EN World D20 Modern Forum