On-the-fly data encryption software. Creates virtual encrypted volumes that can be mounted as usual disks with drive letters. TrueCrypt can encrypt an entire hard disk partition or device such as USB memory stick, floppy disk, etc. Supported encryption algorithms: AES-256, Blowfish (448-bit key), CAST5, Serpent (256-bit key), Triple DES, and Twofish (256-bit key). Cascading is supported (e.g., AES-Twofish-Serpent). Also, TrueCrypt enables you to create a hidden encrypted volume (a TrueCrypt volume that sits within another TrueCrypt volume).
A TrueCrypt volume is impossible to identify: until decrypted, a TrueCrypt volume appears to consist of nothing more than random data (TrueCrypt does not leave any kind of "signature" like many other encryption programs do), therefore, it is impossible to prove that a file, a partition or a device is a TrueCrypt volume and/or that it has been encrypted. TrueCrypt containers (file-hosted volumes) can have any file extension you like (for example, .raw, .dat, .iso, .img, .rnd) or they can have no file extension at all.
Note: "Download" links to the TrueCrypt Downloads page.