Prerequisites for Outlook 2007 (see 2010, 2013 below)
OutlookGnuPGHere is the Github project : GnuPG addin for Outlook 2007
NOTE: To get this to work in Vista/7 you have to run the installer as administrator. Find your command shell in the run menu, right click and select run as administrator. Then do msiexec -i "path\and\filename.msi". That should get you past the impersonation difficulties.
PGP for Outlook 2010, 2013A simple OpenPGP encryption plugin for Outlook 2010 and 2013.