Twofish is a block cipher by Counterpane Labs. It was one of the five Advanced Encryption Standard (AES) finalists. Twofish is unpatented, and the source code is uncopyrighted and license-free; it is free for all uses. The designers of Twofish offered $10,000 in prize money for the best attack on Twofish during the first round of the AES evaluation. Congratulations to Fauzan Mirza and Sean Murphy, who won the contest for their paper "An Observation on the Key Schedule of Twofish."
See the twofish homepage for more details.