- LibreSSL
- BearSSL
- mbedTLS (PolarSSL)
- MatrixSSL
- WolfSSL (CyaSSL)
- NanoSSL
- SharkSSL
- Golang crypto/tls
- Java Secure Socket Extension
- Java/C# BouncyCastle
- C++11 Botan
- Rust Rustls
- Google BoringSSL
- Amazon s2n
- Mozilla NSS
- Microsoft SChannel
- Apple Secure Transport
- RSA BSAFE
- GnuTLS
- OpenSSL
- Netflix msl
- ZeroMQ CurveZMQ
- ...Many many more...
- NaCl/libsodium
- Cloudflare CFSSL
https://github.com/gpg/ntbtls used by gpg
Also a Wiki page has some more libraries
https://en.wikipedia.org/wiki/Comparison_of_TLS_implementations