A few applications can be used to encrypt data conveniently on your personal computer. (This is by no means a complete list of every application.)
Pretty Good Privacy (PGP) is mentioned in this guide because it is a useful protocol suite. Created by Philip Zimmermann in 1991, it passed through several versions that were available for free under a noncommercial license. PGP applications can be plugged into popular e-mail programs to handle the majority of day-to-day encryption tasks using a combination of symmetric and asymmetric encryption protocols. One of the unique features of PGP is its ability to use both symmetric and asymmetric encryption methods, accessing the strengths of each method and avoiding the weaknesses of each as well. Symmetric keys are used for bulk encryption, taking advantage of the speed and efficiency of symmetric encryption. The symmetric keys are passed using asymmetric methods, capitalizing on the flexibility of this method. PGP is now sold as a commercial application with home and corporate versions. Depending on the version, PGP can perform file encryption, whole disk encryption, and public key encryption to protect email.
TrueCrypt is an open source solution for encryption. It is designed for symmetric disk-based encryption of your files. It features AES ciphers and the ability to create a deniable volume, encryption stored within encryption so that volume cannot be reliably detected. TrueCrypt can perform file encryption and whole disk encryption. Whole disk encryption encrypts the entire hard drive of a computer, including the operating system. FreeOTFE is similar to TrueCrypt. It offers "on-the-fly" disk encryption as an open source freely downloadable application. It can encrypt files up to entire disks with several popular ciphers including AES.
GnuPG or Gnu Privacy Guard is an open source implementation of the OpenPGP standard. This command line-based tool is a public key encryption program designed to protect electronic communications such as e-mail. It operates similar to PGP and includes a method for managing public/private keys.