From the Wikimedia Technical Blog (blockquotes omitted):
If you’d like to read Wikipedia in an airplane (of the offline variety) or in an area with no or limited connectivity, or install it in a university, or just to have it handy in case of a zombie apocalypse, you can now download a full text copy of the English Wikipedia (from January 2012) in the convenient OpenZIM format, which was specifically developed for sharing wiki content.
Start your BitTorrent client and grab a copy of the 9.7GB file (.torrent link, other download options). You can also download content packages directly from within Kiwix using its library feature, including content from sister projects like Wiktionary and Wikisource, as well as non-Wikimedia content.
While the ZIM file doesn’t include images (that would blow it up to ~100GB for thumbnail-sized images), it does come with all the lists, tables, citations, and even mathematical formulas included in the online version.
Please note that this OpenZIM file was prepared by Emmanuel Engelhart, the developer of Kiwix, and feedback should be directed to him (contact at kiwix dot org) or submitted through the Kiwix feedback system.
(Via Wikimedia Technical Blog. Licensed under a Creative Commons Atribution 3.0 Unported License.)