TrueCrypt is a free software that encrypts data “on-the-fly”. 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 doesn’t simply encrypt 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.