There is a German, a Dutch, a Norwegian, a Polish, a Portuguese, a Slovak, a Slovenian, a Spanish and a Greek version available.
Apart from that the data is also available in formats for use with the word processors of the office suites LibreOffice and OpenOffice.org (Writer) or as a complete database dump.
[1] The cause for the start of the project was the arrival of OpenOffice.org in 2002, which was missing the thesaurus of its parent, StarOffice, due to its licensing.
OpenThesaurus filled that gap by importing possible synonyms from a freely available German/English dictionary and refining and updating these in crowdsourced work through the use of a web application.
[2] The project has gained a lot of popularity following the arrival of Apple Dictionary in Mac OS X 10.5, which can integrate OpenThesaurus data through a plugin.