Palladium

Palladium Books is a company mostly ran and founded by Kevin Siembieda. It has produced around 15 seperate settings all using a single semi-standardized game system. It is about on the same level of complexity as D&D 3.5, but may be harder to learn from the book because of layout (depending on the which book you use, some are wierder then others, opinions fa/tg/uys?).

Settings:

After the Bomb Beyond the Supernatural (CoC esque) Chaos Earth Dead Reign Heros Unlimited Palladium Fantasy RPG Nightbane Ninjas and Superspies Rifts Robotech Mechanoids Recon Splicers

As well as several others, and a few non-setting specific series such as the wepon compendiums (made to be at least usefull, if not eaisly adaptable to any system), and the quarterly released Rifter (a hodgepog of material that never made it to the source books along with homebrews Kevin made cannon).