First is a revised assassin class for S&W Complete by Chuck Barchuk, which i think has a great and unique take on poison use. Also, here is a direct link to the PDFs – there is a percentile version and simpler d6-based version.
Next is a handy set of house rules by forum user ratmanof to make S&W White Box a bit more like Greyhawk with variable hit dice, a simplified thief class and greater ranges to weapon damage.
My friend John of Damn Elf Press fame also has a long-running and amazing White Box:FMAG play-by-post “Beyond the Borderlands”. Stop by if you like that sort of thing, and keep an eye out for player openings. Even if you don’t enjoy gaming by PbP, it is entertaining reading!