TrueCrypt Disk Encryption Software
TrueCrypt is free open-source disk encryption software for Windows Vista/XP, Mac OS X, and Linux. It provides transparent on-the-fly encryption. You can create and mount encrypted drives — either as separate files or entire partitions. TrueCrypt uses AES-256, Serpent, and Twofish encryption.
TrueCrypt has the ability to create encrypted volumes inside one another. Since a TrueCrypt volume looks just like random data, this gives you plausible deniability: You may be forced to reveal the password to your outer TrueCrypt volume, but there is no way to determine whether you have a hidden TrueCrypt volume within it.
TrueCrypt supports encryption of an entire system drive. For example, you can encrypt the drive where Windows is installed and from which it boots. And the strange thing is that despite the encryption overhead, it may actually speed up your system — see “Security Now! Transcript of Episode #133” for details.
TrueCrypt version 6.1 was just released a few days back.