The GPGServices service plugin adds to the global Services menu (in the Application menu) the facility to sign and verify plain text in any application that supports text services. The plugin also provides convenience facilities to import an ascii-armoured key from text in any application as well as return your default key, in ascii armoured format, and the key's fingerprint.
Signing and pasting your key or fingerprint require the host application to support pasting of text (so you will not be able to sign text highlighted in a pdf file in Preview, for instance). Verifying a signature and importing a key only require that you can highlight the text.
To install the service, download and unzip the file linked below and copy the GPGServices.service into the Services folder in your system Library folder (the one you see if you click on Macintosh HD). If no Services folder exists, create one. Then you will need to logout and log back in again for the service to be recognised. If you wish to use the My Key and My Fingerprint options, you will need to set your default key ID in the gpg preferences file (or via the very useful GPGPreferences System Preferences pane).
To use the service, as an example; open TextEdit and type some text and highlight this text. Then select the TextEdit menu -> Services -> GPG and select the Sign option. You will be asked for your GPG passphrase. If you enter the correct passphrase, your text will be replaced with a signed copy, which you can then highlight and verify using the Verify option. This will work just the same for webmail or forums as it will in TextEdit.
Please note that the GPGServices service is a Beta product. Bug reports, comments and feature requests are always welcome - please email.
Source built using Xcode 2.2. You will need to download the MacGPGME Framework separately. GPGServices is Universal.