TrueCrypt is an open-source encryption software that enables you to create a virtual encrypted disk within a file and mount it as a virtual disk, that can be accessed via a drive letter. Any file that is stored on this virtual drive is automatically encrypted on-the-fly, and can only be accessed while the drive is mounted with the correct password or key. TrueCrypt supports a variety of encryption algorithms, including AES-256, Blowfish (448-bit key), CAST5, Serpent, Triple DES, and Twofish. Other features include support for FAT32 or NTFS formatting, hidden volumes, hotkeys for mounting/dismounting and more.
Pros: Can create hidden password protected and encrypted partitions in your hard drive, up to goverment encryption standards, too many features to list here, the best free encryption software on the net.