I have been using a great utility Cryptext from Nick Payne since long ago. The utility is a shell extension that encrypts/decrypts selected files with a password using well known cryptographic algorithms and even has sample source code to derypt included. A fool proof check for correct password and an option to keep the password in memory until shutdown is included, it is great… I am using version 3.40. There already is no homepage of the utility and the author but it still works fine, expect one problem I stumbled upon just recently. It failed to process a large file, larger than 2^31. Oops!