Crowdin - Localize your product in a seamless way
Weblate - Weblate is a free web-based translation management system.
Poedit - Poedit translations editor. The best way to translate apps and sites.
Transifex - Transifex makes it easy to collect, translate and deliver digital content, web and mobile apps in multiple languages. Localization for agile teams.
POEditor - The translation and localization management platform that's easy to use *and* affordable!
Pootle - Multiple file formats.