This is the perfect compliment for anyone using GPG, Gmail, and Firefox. I use it on Windows as well as in OS X. This Firefox plugin integrates PGP signing, encryption, decryption, and verification all in the Gmail web interface. Buttons are added alongside the standard Send, Save Now, and Discard buttons for Sign, Sign and Send, Crypt, Crypt and Send, Sign and Crypt, and- you get the idea; a button for every possible combination. Signatures are automatically verified if present against public keys in your keyring and you can decrypt messages while viewing by clicking a 'Decrypt this email' link.

Installation and setup are easy. Install the plugin, then go into the plugin preferences and make sure it has the correct path for GPG.

Get it here.