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GPG works on OS X, Linux, and Windows with extensive selection of wrappers.
Pro Multiple types of encryption algorithms
GPG supports public key cryptography (RSA EIGamal, DSA), symmetrical key algorithms (Blowfish, AES, IDEA, etc), cryptographic hash functions (RIPEMD, SHA), and compression (ZIP, ZLIB, BZIP2).
Pro Excellent but...
Open source (the free version of openPgp), other similar products are offered for other platforms (Gpg4win for Windows for example). It is the most secure way to send emails today, close to military encryption. Some negative points: not all mail clients support it and when a key is not only on your hard disk but also on a server (pgp.mit.edu for example), it is complicated to remove it.
Last positive point: it can also be used other than for emails, to encrypt and sign documents. Bitcoin releases are signed with the GPG keys of the developers ;)
Pro Supports paired keys
Allows for encrypted communication.
Pro Volume and individual file encryption
With GPG you can encrypt you whole volume or files individually.
Pro Supports expiring signatures
GPG keys by default expire after a set amount of time. The amount can be changed and this feature can be turned off.
Pro Both CLI and GUI versions available
GPG can be installed as a command line tool, or you can choose between several different GUI frontends available for it.
Pro Open-source and battle-tested
GPG is the oldest and most reliable encryption software available.
Con It may be hard to find a GUI frontend that suits your needs
If you decide not to use the CLI version of GPG, it may be hard to find a GPG GUI version that suits your needs simply because of the sheer number of different versions available.