Articles: Wikipedia with class
If you frequent Wikipedia -- and appreciate an interface that's both beautiful and highly functional -- Articles is an iPhone (and iPad) app you should try. Going beyond just being a wrapper for the less-than-beautiful compendium of user-generated knowledge, it offers a very pleasing way to navigate the tomes.
In addition to normal browsing, Articles offers integrated maps and uses geolocation to offer items of interest around you, or around any location you specify. It also has support for multiple pages, a la Mobile Safari, so you can follow multiple lines of research without losing your place. Automatic spelling suggestions help you locate your information, and an integrated chapter browser helps you navigate. You can bookmark pages, and organize bookmarks in folders, as well as search your history by day. Recently-viewed articles are even available offline.
I haven't played with every Wikipedia app out there, and I've heard some great things about others like Wikipanion. Within my experience, though, Articles is a very complete solution and definitely worth a look. Articles is available for both iPhone ($2.99US) and iPad ($4.99US). I've only had the opportunity to play with the iPhone version, which is beautiful, but I'm really looking forward to trying it on the iPad, where its gorgeous interface and fluid usability will really have a chance to shine.