The list below are specialized softwares for PO files with support like message sorting, table view, fuzzy toggle button, statistics, PO header support, plural forms…

• Poedit (wxWidgets)
• Gtranslator (GNOME)
• Lokalize (KDE)

Text based editors with PO extension:

• VIM with po.vim plugin
• Emacs with po-mode

Mousepad, Leafpad, or any other notepad-like application will do but they aren't recommended.