Poedit is free software application that allows edit gettext catalogs.
Poedit is cross-platform software and it is open source. It allows to edit *.po files and update them from *.pot catalogs file. It is used in multilingual applications that uses gettext.
As an example for poedit usage, the internationalization features of cakephp, in-which the author run command line script that extracts all gettext strings from the application files in *.pot file, then using Poedit, the author edit the terms in the file then saving it as po file in the local directory.