Mobile Interface Toolkits
These toolkits are currently free* and provide many styles and effects for developing mobile websites for Webkit based mobile browsers. From a .NET perspective they are particularly effective to use with ASP.NET MVC style development.
These are frameworks that you would use instead of fully fledged libraries such as jQuery that are optimised for cross browser compatibility on Mobile Browsers. Generally they are much smaller because there is a lot of code specific to desktop browsers in the traditional libraries.
ASP.NET Webforms Toolkits
For those looking to jump start into iPhone and Andriod web development, the toolkit from ComponentOne can provide the familiar Webforms development experience and render the much loved iPhone styled mobile websites.