XowA does one thing and does it well: It lets you make a local backup of Wikipedia (probably any Mediawiki, really) to carry around with you or copy to removable media (like a USB key or DVD-ROMs) so you can give out copies and read it offline. Runs on Windows, Linux, and MacOSX. Portable - can be run from removable media. Even includes a search engine. Written in Java.
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