Cypher System and Chaosium Basic Roleplaying have actual OGLs.
Savage Worlds has several levels of permission/licensing, though not an actual OGL; they have tiers for fans and small creators to use their system without a formal license, as well as offering licensing of their system to larger publishers.
It’s not clear to me if Powered by the Apocalypse has an OGL, but it does sound like the creators encourage others to use and hack their system.
Also, for what it’s worth, Fate also has an OGL.