libsodium
Modern crypto library: portable fork of Bernstein's NaCl. C + bindings to 25+ languages. High-level API that makes it hard to use incorrectly. Curve25519, Ed25519, ChaCha20-Poly1305, BLAKE2.
Frank Denis maintains since 2013. Default in Tox (P2P chat), Mullvad, many password managers. Distinct from OpenSSL: fewer features, better defaults, less history burden. crypto_box_easy() gives NaCl-style authenticated encryption — public-key encryption in one function. Constant-time guarantees, dragonblood resistance, formally verified primitives. Competitor: BoringSSL (Google, more TLS focus), Tink (Google, multi-language API), monocypher (single-header alternative).