One of my main uses of my iPhone is accessing information on the Internet. When I'm out with friends they've sort of gotten in the habit of expecting me to look up information whenever a question comes up in conversation. Anything from wanting to know the lowest-paid team in baseball (the Marlins) to knowing whether pomegranate is a citrus fruit (it's a fruit-bearing shrub).
My application of choice when I'm called upon is often Wikipanion, because it gives such quick and convenient access to Wikipedia, the infamous encyclopedia that anyone can edit. Wikipanion, which is free, does a great job of presenting all of the elements of Wikipedia articles — as if it has disassembled the page and put it back together again for your iPhone.
The Smart Completion feature predicts what key words your entering as you type them. You can bookmark articles for later reference, and even sections of articles. The settings let you govern a number of variables, including text size and default language.
One neat feature is an icon on the screen that lets you pop up and view the table of contents no matter where you are in the article. Another is the ability to search within an article.