Developer: Robert Chin
Price: free Download on the App Store
The top app on the iPhone is a search app, and it's not Google. Wikipanion is a simple, clean version of Wikipedia for the iPhone; and its also very popular - more so than any free game, and more so than the king of search, Google.
Despite the latest attempts from Google to compete in the user-contributed content space (does anybody even still remember Knol?), Wikipedia's scale advantage seems to be growing every day. It's a testament to the usefulness of the service that even Google ranks Wikipedia results towards the top of most searches.
The app itself is well done. Unlike the Google Mobile App (now called Google Search), the fonts are always perfectly-sized to fit the iPhone screen. The organization actually seems slightly cleaner for the iPhone than for the web itself. And the content is irreplaceable.
If you search on your iPhone, you will want both Google Search and Wikipanion, and if you are like me, you will end up using Wikipanion. A lot.