True crypt is a free open-source disk encryption software for Windows Vista/XP/2000 and Linux. Main Features: Creates a virtual encrypted disk within a file and mounts it as a real disk. Encrypts an entire hard disk partition or a storage device such as USB flash drive. Encryption is automatic, real-time (on-the-fly) and transparent. Provides two levels of plausible deniability, in case an adversary forces you to reveal the password: 1) Hidden volume
2) No TrueCrypt volume can be identified (volumes cannot be distinguished from random data).
Encryption algorithms: AES-256, Serpent, and Twofish. Mode of operation: LRW. Learn more and download here