The Japanese language branch of WikiPedia has been around at least since December 2001. It was converted to the WikiPedia PHP code base in September 2002, using the Unicode/UTF-8 character set, but as of March 2003 still only had 250 articles. However, that spring was the start of its explosive growth. In December 2003 it passed the FrenchWikipedia, becoming the third largest Wikipedia after English and German and perhaps the 4th BiggestWiki anywhere. The Japanese Wikipedia has an article count growth rate approximately the same as the German Wikipedia, and a large contributor base.
For a note on how to read the site statistics, see the ChineseWikipedia.