From Nexus D20 Wiki
Nexus D20 Player's Guide - This contains the latest set of rules for Nexus D20.
Nexus Low Tech Campaign Book - This is the GM guide for running low tech games in Nexus D20.
Nexus is a universal game system based on the d20 game system but with a lot of changes to make it comprehensive and faster to play and easy to run. Through it you can run any world or genre you want. Several different genre settings are laid out. You can have characters cross over from any genre that your GM wants to allow. This system can be used for a single game or a multiverse game. You can use it to play this setting, your own setting, or even published settings that you like but don't like their system. It is all up to the GM and the players. Have fun with it.
All references to other game systems and settings are used under fair use. All Trademarks are owned by their respective companies and this is not a challenge. If any reference or picture use is disputed then contact me and I will immediately remove it. firstname.lastname@example.org
OGL Legal - Nexus D20 is published under the Open Gaming License.