GooSig reference code
https://github.com/kwantam/GooSig

Handshake's GooSig implementation
https://github.com/handshake-org/goosig/

Systemization of Knowledge: Computer-Aided Cryptography
https://eprint.iacr.org/2019/1393
An overview of current efforts at provable cryptographic software.

Fiat Crypto
https://github.com/mit-plv/fiat-crypto
Work out of Adam Chlipala's group at MIT on generating certified
implementations of crypto software. Now used in BoringSSL!

miTLS
https://mitls.org/
A certified implementation of TLS. There's also been plenty of work
since then by (some of) the authors on proving security of the TLS 1.3
protocol, e.g.,
https://www.microsoft.com/en-us/research/publication/implementing-proving-tls-1-3-record-layer/

HACL*
https://github.com/project-everest/hacl-star
A formally verified cryptographic library, used in Mozilla's libNSS.