Crowdin - Localize your product in a seamless way
POEditor - The translation and localization management platform that's easy to use *and* affordable!
Poedit - Poedit translations editor. The best way to translate apps and sites.
Weblate - Weblate is a free web-based translation management system.
Translate Toolkit - The Translate Toolkit is a localization and translation toolkit.
gted - gted (GetText EDitor) is an editor for gettext po files.