poEdit is cross-platform gettext catalogs (.po files) editor. It is built with wxWidgets toolkit and can run on any platform supported by it (although it was only tested on Unix with GTK+ and Windows). It aims to provide more convenient approach to editing catalogs than launching vi and editing the file by hand.
This is an editor for editing .po files, which are wildely used for tranlating text in various software. I don’t find any nicer editor for this purpose on the Mac platform. As far as I know, Drupal tranlation requires editing .po files.
Check out the GNU project page for gettext for further information.
Update: TextMate does have gettext bundle. But, I wanna see a dedicated Mac app for this purpose.