There are many ways to download and read Wikipedia articles offline, some of which has been mentioned in the past. There are standalone tools like Okawix and WikiTaxi, and browser add-on Zipedia that lets you download and read Wikipedia within Firefox. Not long ago, a Chrome extension was also developed that does the same for Google Chrome browser. Raw database dumps of Wikipedia articles has long been available for anyone who wants to set up a Wikipedia clone on their personal computer.
Recently, the Wikimedia Foundation announced a new way to download and read Wikipedia offline. The full text copy of the English Wikipedia (from January 2012) is now available in the convenient OpenZIM format, which was specifically developed for sharing wiki content.
The ZIM file format is an open, standardized file format to store Wiki content efficiently for offline usage. Its primary focus is on the contents of Wikipedia and other Wikimedia projects, but can be used for any wikis. The format allows for the compression of articles, features a full text search index, and native category and image handling similar to MediaWiki.
OpenZIM files can be read in multiple reader applications, the most popular of which is Kiwix, available for Mac, Windows, and Linux.
Start your BitTorrent client and grab a copy of the 9.7GB file. The copy was put together by the developer of Kiwix but any application capable of handling a OpenZIM file can use the copy. 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.
This copy contains only the text articles along with lists, tables, citations and even mathematical formulas. Images and other media files are not included though. Including just the thumbnail images for all the articles would have swollen the archive to more than 100 GB.
According to Softpedia, this is the first full copy of the English Wikipedia. A previous attempt only archived 45,000 articles, a small fraction of the total, and dates from 2010.