Libraries for freestanding C++ development:

• https://github.com/osdev0/freestanding-cxx-hdrs-gnu
• https://github.com/etlcpp/etl
• https://github.com/managarm/frigg
• https://github.com/teslamotors/fixed-containers
• https://github.com/Bareflank/bsl

Not freestanding, but interesting:

• https://github.com/woven-by-toyota/arene-base