Blowfish

Bruce Schneier designed Blowfish as a deliberately unpatented, freely available alternative at a time when DES was on its way out and most replacements carried licences. It is a Feistel cipher with 16 rounds, 64-bit blocks and keys from 32 to 448 bits.

The key schedule is unusually expensive: every new key requires 521 runs of the cipher to fill the S-boxes. That was intended as an obstacle to exhaustive search, and instead became the foundation of the bcrypt password hash, still in use everywhere. The cipher itself is obsolete — 64-bit blocks make it vulnerable to Sweet32 — and Schneier now points people at Twofish and AES.