Defensive Patent License (DPL)
The copyleft idea applied to patents: share everything with everyone who also shares everything.
Jason Schultz and Jennifer Urban at UC Berkeley's law school presented the license in 2013. Anyone who joins offers all of their patents royalty-free to every other DPL user – not a selection, the entire portfolio. Sue someone in the network, or leave it, and you lose access to other members' patents going forward, while licenses already granted stay in force.
It was designed for researchers, startups and non-profits that file patents defensively but never intend to wield them. It has stayed a niche: large portfolio holders prefer to choose for themselves, and network effects are hard to bootstrap with a license that only pays off once plenty of others have already joined.