Open Game Systems

This page collects games released under an open license. Generally speaking, if something is available under an open license it is free for use and re-use as long as the terms of its license are respected, without any explicit contact or negotiation between original author and licensee. Requirements vary from offering no restriction whatsoever to requiring that you credit the authors or that you also allow others to use your own derivative work freely.

More on the RPGnet wiki.

https://wiki.rpg.net/index.php/Open_Game_Systems