Serpent is an encryption algorithm designed by Ross Anderson, Eli Biham and Lars Knudsen as a candidate for the Advanced Encryption Standard AES competition, where it got second place with 59 votes (Rijndael got 86 votes, and was selected by NIST as the AES).
Serpent uses a block size of 128 bits and supports a key size of 128, 192 or 256 bits.
Serpent and Rijndael are somewhat similar. The main difference is that Rijndael has fewer rounds (10, 12 or 14 (depending on key size) compared to 32 for Serpent), hence it is faster. Arguably, Serpent is more secure.
Serpent is available as public domain, and can be freely used by anyone.