If you have Safari on your computer, you can enable the "Develop" menu under Preferences > Advanced > Show Develop Menu in Menu Bar.
With this enabled, you can go to Develop > User Agent, and change the user-agent string to the device you want your browser to report to the web server as.
By resizing the window to the appropriate width, you can emulate what the site will look like on the iPhone.
The upside of this is that it's quick, it works on both Windows and Mac, and you don't need the iPhone SDK installed. You can also browse iPhone-specific versions of websites that catch user-agent strings directly from your PC.
The downside is obviously your Safari browser on your PC will behave quicker than on the actual device (especially in regard to javascript performance); it displays plugins and shows fonts that may not be available on the actual iPhone OS; a lack of multi-touch support and "snapping" to columns while scrolling; no auto-rotation; no multi-touch/pinch-zoom; widgets will look different; etc.