TODO (Feel Free to add information, please indicate if it is Free Software or not and if it uses the recommended GPGME and uiserver APIs of GnuPG)

Enigmail

https://www.enigmail.net

a data encryption and decryption extension for Mozilla Thunderbird and the SeaMonkey internet suite that provides OpenPGP public key e-mail encryption and signing.

License: MPL 2, GNU GPL 2+

API: no GPGME, no UIServer

gpg4usb

http://www.gpg4usb.org seems to use GnuPG 1.4.x and Qt.

portable-application, which combines a simple text-editor with a GnuPG-frontend to write, encrypt and decrypt your text-messages and files.

API: Version 0.3.3 may still use GPGME. The TODO file aims for the use of its own kgpg connection. no UIServer

duplicity

Duplicity backs directories by producing encrypted tar-format volumes and uploading them to a remote or local file server. [..] uses GnuPG to encrypt and/or sign these archives

License GNU GPL v>=2.

API: uses its own embedded gpginterface.py, process based, does not use GPGME