are open source encryption packages.
TrueCrypt is based on E4M and is disc based encryption. Everything is encrypted... you have a file that you mount as a disc volume. You can create a volume of about 650MB (or 4.3GB for DVD) and burn it to the disc. There are a ton of options. You can include TC on the disc and have autorun mount a volume in RO mode, or just mount it yourself (best choice) so no one knows how to open it.
GnuPG is based on OpenPGP. It is primarily used as individual file encryption, but often our clients have us zip multiple files into one large file and we PGP encrypt that. PGP is PKI based encryption and has a very large user and corporate user base. PGP products are available in open source and pay/closed source software (i.e. PGP Corporation CLI or GUI, Authora Egde, Veradis FileCrypt, McAfee e-Business Server). Probably the most popular way to encrypt data.
Although not used much in the corporate world 7-zip
(open source file compression) supports AES-256 archives and they can be burned to disk with a SFX.
Additionally, PGP Corp has it's own PGP WDE (whole disc encryption) product that can encrypt an entire partition or container file. The container file is much like TrueCrypt... WDE is for the whole hard disc... not practical for optical drive use. Personally I'm not fond of PGP Corps offerings.
This is a 30,000 foot overview.