Wikipedia just release a mobile version application for the iPhone so you easily read and browse threw their millions of articles.
The description of the application clearly said that the applications is 100% open source and community built. So if you are a programmer you can help them out developed the application even further.
This application is base on being very simple and very fast. According to the description more features will come with time.
This application is available for the iPhone and iPod touch second generation and it requires the firmware version 3.0 or later
The application is completely free on the app store you can get it here.