Marketcircle has released a Mac OS X application that simulates the size of the iPhone's screen, offering a built-in WebKit-based browser. Dubbed "iPhoney," the application can simulate the iPhone user agent, rotate to display pages vertically or horizontally, and show or hide the location bar. It cannot, however, simulate the actual WebKit limitations of the iPhone (no Flash, no Java, etc.)
Sites are rendered in a 320 by 480-pixel (vertical) or 480 by 320-pixel (horizontal) window, with a stylized iPhone frame. It's pixel accurate, meaning that it has exactly the same number of pixels on its screen as iPhone. However, while common desktop screens deliver 72 and 96 pixels in every inch the iPhone will pack 160 pixels per inch. So iPhoney looks about twice the size of iPhone on a standard Mac OS X system.
The developers of the application say:
"iPhoney only simulates web browsing in the visual shell of an iPhone. It's not meant to simulate the iPhone experience. [...] Unfortunately, right now, you can't pinch and zoom. However, we're still working on iPhoney and will be making it open source soon. Maybe this feature will find its way in, so keep checking back. [...] Some features, like bookmarks and multiple screens (browser windows) aren't working in version 1.0. We've kept the buttons there so it looks like iPhone, and disabled them appropriately."