TrueCrypt is a free open source on-the-fly encryption program for Microsoft Windows XP/2000/2003/Vista and Linux oses.
Encryption algorithms supported by TrueCrypt include AES, Blowfish, CAST5, Serpent, Triple DES, and Twofish. It also allows the use of a cascade of different ciphers, for instance AES+Twofish+Serpent.
From the article:
Right now the newest version released is version 4.3. You can create an encrypted hard drive, a separate partition or a directory with TrueCrypt. It doesnâ€™t encrypt simply the content of files but their names and the names of the directories they are in as well. Moreover there is no way to check the size of the encrypted directory/HDD/partition. TrueCrypt is available for Windows and Linux.