Crowdsourced Localization

Software products and online content
available for crowdsourced translation and localization

Open source, non-profit:
Wordpress (blogging tool and a content management system)
Joomla (content management system for publishing content on the web)
MediaWiki (interface of Wikipedia and other Wikimedia products)
OpenOffice (alternative to proprietary office suites like Microsoft Office)
Firefox (browser)
7Zip (file archiver, alternative to proprietary WinZip)
Unix-related:
  Fedora
  GNOME
  RedHat
  Ubuntu
  Ubuntu Launchpad

Non-profit, but not open-source:
Notepad++

For-profit:
Facebook
Twitter
Skype
Whatsapp (67 languages as of 2012-11-30)
Netvibes
Go SMS (Android app, 50 languages)
eBuddy messenger (59 languages)

Popular products not open for translation :
Android
Gmail
YouTube
Chrome
Google Docs
Windows
Microsoft Office

Content available for translation:
Amara (formerly Universal Subtitles) http://amara.org (used by TED.com)

Crowdsourced translation and localization tools:
TranslateWiki (only used by Wikimedia products such as Wikipedia)
Google Translator Toolkit (helps translate any text using Google Translate suggestions, has optimized modes for translating Wikipedia and Google products)
Amara (formerly Universal Subtitles) http://amara.org (used by TED.com)
Narro
Transifex

Sites and resources for organizing crowdsource translations: Quora answer