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.

Note: GPG tools [http://www.tomsci.com/gpgtools] is a closed source equivalent to this project, even if the author is prepared to provide the source code there will always be a trust question regarding the distribution binary.