SkyWater's 130nm has been used for all (most?) of the Efabless × Google MPW (multi project wafer) runs. That PDK was open sourced as part of that initial effort.
Most PDKs have an NDA, but Skywater is an exception. Also, most PDKs have some software restrictions on their standard cells because they never tested them with the open-source tools, but with difficulty you can use their design rules in software like MAGIC.
