Translatewiki.net

Translatewiki.net is a web-based translation platform powered by the Translate extension for MediaWiki. It can be used to translate various kinds of texts but is commonly used for creating localisations for software interfaces.
It has about 12,000 translators and for over 5,800,000 pages from over 60 projects including MediaWiki, OpenStreetMap, Mifos, Encyclopedia of Life and MantisBT.
Features.
Translatewiki.net is a wiki and so has a relatively low barrier to entry.
Translations are synchronised between a version control system and translatable wiki pages. 
For MediaWiki on Wikimedia Foundation projects, new localisations may reach live sites within a day.
The translation editor provides various features for machine-assisted translation, such as
Translatewiki.net is also a Semantic MediaWiki, part of the semantic web.
History.
Translatewiki.net was made available by Niklas Laxström as localisation platform for all languages of MediaWiki around June 2006, when it was named Betawiki.
Besides translation, it was developed with the characteristics of an integrated development environment for MediaWiki (Nukawiki in 2005), with a focus on improvement of internationalisation features.
At the end of 2007 Siebrand Mazeland joined the management of the website, which was moved to the current domain .
In April 2008, it already supported over 100 languages for MediaWiki and 200 of its extensions, "making it one of the most translated software projects ever", as well as FreeCol. Since then, while being an independent volunteer project, it has been recognised as a major player in the global success of MediaWiki and the Wikimedia projects powered by it, like Wikipedia, in over 280 languages.
In 2009 it was improved by a Google Summer of Code project by Niklas Laxström.
In 2011 proofreading features were introduced.
In 2012, its translation memory engine expanded to all Wikimedia projects using Translate.
In 2013, the Translate platform underwent a major revamp through the "Translate User eXperience" project, or "TUX", including "changes in navigation, editor look and feel, translation area, filters, search, and color & style".
Supported formats.
Some of the natively supported formats follow. More can be added with some customisation.