XOWA is an interesting Java-based tool which allows you to view and edit a subset of Wikipedia's pages while offline.
Launch the program initially and an opening page provides a link to download and install Simple Wikipedia. This is a limited version of the real thing, but still manages to include more than 95,000 pages, more than enough to be useful.
If you'd like to see images, though, there's more preparation to be done, as the program must also install ImageMagick and Inkscape. The process is at least largely automated, though - just click a link and XOWA downloads the necessary files and unzips them appropriately. This isn't quite as smooth as it should be - when we tried to view pages afterwards, we got a Java error message - but after everything worked fine after we restarted our PC (and this is an alpha, so occasional problems are to be expected).
Once you have got everything set up, though, XOWA works very well. Page formatting is excellent; it looks and feels just like the real thing. Navigation is easy, with links working just as you'd expect (as long as they're to pages included within Simple Wikipedia, anyway). Simple navigation tools include bookmarks and a page history, and you can even edit your local pages.
What's more, if you really need more offline resources, then the program can link those in, too. So while editing a page in your offline Wikipedia, for instance, you could in theory look up the spelling of a word in your offline Wiktionary.
There are some oddities here. As you're not working in a browser, for instance, XOWA provides a "forward" and "back" button of its own. But these are so small that it took us a while to even notice them. Hopefully clearer navigation is on the list of "things to do".
Issues like this are of course very minor for an alpha build, though, and in general XOWA performs very well indeed. If you need a portable and cross-platform mini-Wikipedia which you can use anywhere, then go grab a copy immediately.
Despite a few minor issues (no surprise for an alpha build), XOWA is already an excellent offline reader for Wikipedia