See http://openzim.org/Main_Page and http://www.wikimedia.ch/Main_Page for more information.
Quote: "updated DVD based on free software
Wikimedia CH has released a new edition of the German Wikipedia DVD on LinuxTag 2009 from 24th to 27th of June, in collaboration with openZIM. The DVD contains more than 900,000 German Wikipedia articles and a full text search index for phrasal search. The openZIM project develops a file format called "ZIM" to store hypertexts like Wikipedia, or other websites, with search indizes and images in the most efficient way. The data is highly compressed; the Wikipedia articles only take up 1.4 GB on the DVD."