TrueCrypt

This is an open source project which enables the user to create an encrypted disk image (effectively a file which appears as a disk to your computer). This enables secure storage of sensitive information. The operation is relatively straight forward and the learning curve for new users is very small.

It supports command line and GUI operation as well as password-only and certificate based authentication.

This can be used for keeping all of your information secure or simply sending a collection of sensitive files to someone over the internet.

The project also supports "whole disk encryption" which is mostly used for laptops to prevent a stolen laptop from turning into a security breach. I have NOT tested and therefore cannot vouch for this feature myself.