iPhone/iPod touch only: Free iPhone app Wikipanion searches Wikipedia through a fast, native interface. Complete with autosuggest, landscape mode, and large, readable text, Wikipanion offers a fantastic way to browse Wikipedia on-the-go. The application's bottom row of buttons provide simple navigation (back and forward), links to sections for quick navigation of large articles, and access to every link on the page, including Wikipedia category links or external links. The free Wikipanion is simple, but does exactly what it promises.
Wikipanion [iTunes Store]