Zipedia is a Firefox addon that downloads the entire English Wikipedia to your hard disk and enables you to search and browse articles offline right within Firefox. Wikipedia provides XML dump of it's contents which Zipedia downloads and then processes these XML dumps to be used locally. The download is huge, around 3GB and so it could take a couple of hours depending upon your Internet connection. Once it is done, you can access the wiki by typing "wikipedia://wiki/" in the address bar.
Zipedia rendering of the Wikipedia's pages from the downloaded data is somewhat crude, but it does the job. The formatting is often lost and the page has the same kind of look as a website with a missing CSS file. But as I said, it does the job. Images are missing as they are not a part of the downloaded wiki and has to be retrieved online. Another crippling factor is that key word searching is possible only for words present in page titles. Zipedia won't search for words in the contents. Although it does support wildcard searches.
Zipedia is still experimental and there is a bug which prevents it's use with Firefox 3. It crashes. You will require an earlier versions of Firefox.