http://en.wikipedia.org/wiki/Category:PGP

PGP offers strong protection on file level. GPG is the free implementation.

http://en.wikipedia.org/wiki/Filesys...vel_encryption

Depending on your needs, different levels of protection can be used.

http://en.wikipedia.org/wiki/Disk_encryption

If you want to make a partition or a whole disk protected. For example turn your SD card into an encrypted device. There are several HOWTOs on this.