Localization & i18n Tools
Translation.io
Free for Open Source
transifex.com
Free for Open Source
localizely.com
Free for Open Source
webtranslateit.com
Free up to 500 strings
Texterify
Free for a single user
lingohub.com
Free up to 3 users, always free for Open Source
gitlocalize.com
Free and unlimited for both private and public repositories
oneskyapp.com
Limited free edition for up to 5 users, free for Open Source
crowdin.com
Unlimited projects, unlimited strings, and collaborators for Open Source
weblate.org
It's free for libre projects with up to 10,000 string sources for the free tier and Unlimited Self-hosted on-premises
Free PO editor
Free for everybody
Lecto
Machine Translation API with Free tier (30 free requests, 1000 translated characters per request). Integrated with the Loco Translate Wordpress plugin